Mention was made above (somewhere) of "the big board that slides out from under the cabinet top" or something like that.

In traditional use and intent, this is not a cutting board. It's a "bread board," intended for rolling out the biscuits. Dough products dry enough to be rolled will normally be too dry to support bacterial or mold growth, so sliding them back under the counter doesn't create any particular sanitary hazard (after the crumbs are brushed off, and you've shaken out the sock, of course). Even if you wash them thoroughly after cutting meat, you may be tempted to slide them back in while there's still enough dampness to breed "stuff."

Admittedly, they did (and do) get used as improvised cutting boards; but if you do so you should follow the advice of the 1890s era "Domestic Science" texts and pull them completely out of the slot, lay them on a counter top for use, wash (preferably with chlorine bleach) and dry completely, preferably overnight, before putting them back in the hole.

A better practice is to restrict the "slide outs" to baking, and get a proper cutting board for critter cuisine.

As to the relative safety of plastic versus wood, one thing contributing to the safety of wooden boards is that you do need to keep the surface "fresh" with occasional applications of a suitable oil, and nearly all oils - vegetable or mineral - are naturally bacteriostatic. They don't kill bacteria, but they generally don't support growth. While I personally don't approve of using any mineral oil on wood, the mineral oil/beeswax stuff is fairly commonly used. If you clean and refresh fairly often, a good vegetable oil (I like walnut) should never get rancid on the board. The porosity of the wood also allows it to dry out, and no common bacteria can survive being completely dried. As mentioned above, a cutting board should never be "left wet" longer than necessary to the task at hand.

It is not necessary for a plastic board to be "nicked and scratched" to harbor growing bacteria. The "surface charge potential" of most plastic surfaces suitable for a cutting surface can supply a source of chemical energy that many bacteria can use in metabolic processes with no, or very little, additional "food." The "sticky feel" on that Tupperware that comes down from the top shelf for the church picnic is a direct evidence of how the "surface potential" attracts, holds, and embeds even airborne "particles" and chemicals (think smog?) that can nourish bacteria. Bacteria can and do live on plastic surfaces, and NO NORMAL WASHING can reliably remove them. The chemical film that adheres due to the plastic's surface potential retains the bacteria - and sufficient moisture and food to keep them growing.

On reasonably oiled wood boards, growth of the bacteria is inhibited by the oil, and the wood will dry out until they die. On plastic, the surface potential holds moisture and nourishment and bacteria can live, and sometimes grow.

In order for plastic boards to be "safer" than wooden ones, it is necessary that you have a way to KILL the bacteria, in place, so that it doesn't matter if they're all removed. The ONLY reliable (approved) way of doing this is in a "commercial" dishwasher that exposes the entire board to a temperature of at least 160 F (70 C) for at least 10 minutes. (Restaurant codes in my area say 170 F.)

The health codes' spec temperature is not a "practical" temperature for home dishwashers, but most home washers will maintain at least 140 F for a half hour or so, if they're working properly and are not run on an "environmentally friendly setting;" so they probably can achieve the desired result through longer dwell at a slightly lower temperature. Setting your water system above about 120 F (49 C?) is not recommended due to the risk of accidents in the sink and in the bath, so your dishwasher must have a working heater.

For those without a dishwasher, you can probably "sanitize" your plastic board by slowly pouring a minimum of 1 quart (or about a liter) of boiling water over it for each square foot of surface to be "cleaned;" but this isn't a "code sanctioned" method.

Alcohol is NOT AN EFFECTIVE killer for bacteria on plastic boards, since the same surface potential that binds the bacteria in the surface film (and feeds them) may bind the alcohol, making it ineffective and in effect just giving the bugs more to eat. Chlorine bleach is "approved" as a sanitizing rinse for plastic boards by most health departments I've heard of, but is NOT A SUBSTITUTE for "Pasteurizing" at high temperature. Alcohol does have an effective drying capacity, which may help remove surface moisture; but this is not as effective on common kinds of plastic as on more usual "laboratory utensils" since the "film binding" prevents the alcohol from mixing with all of the bound up moisture.

I do have several plastic "cutting boards," but I use them mainly to "define the work spaces." It seems people are less inclined to pile stuff on the boards than on the rest of the counter tops, so there's usually a place to set down a mixing bowl, at least.

I use wood (almost) exclusively for food preparation that involves cutting - because I like it better as a work surface and I believe it's safe if you treat it with respect.

John