There is sometimes a line drawn between a strict, regemented rhythm by..say..a drum machine, juxtaposed with the ability of music to "breathe" (meaning slight fluctuations in tempo) as adrenaline or energy kicks in during a performance. Both have merits and drawbacks.

For instance, I spent many years in bands, (duos and trios mostly) where we used a drum machine for almost everything performed. It has gotten to the point with me now, where my sense of time is rather acute and I know when practising or playing by myself, if the tempos are solid...But, when I play with others acoustically I can instinctively feel if the timing of the other player(s) is/are dragging or speeding up. The question then arises: Do you point it out to the person(s) in question? I think part of the answer is that if this person strives for perfection or, authenticity in their playing (and you're sure it won't be taken as a "personal attack") then tell them in a diplomatic way, but be very sure of it and have a reference tool (ie: sequencer of drum machine handy) to demonstrate it to them. Chances are they'll thank you for it and be more focussed on it in the future. If they're still finding their way and engaged in an ongoing struggle with not only tempo but in playing correctly, then cut them some slack and allow them to focus on the music. The tempo situtation can be addressed at a later period in time. (In other words let them learn to drive before showing them how to use the "cruise control".)

In a group situation where someone is dragging down the tempo (or speeding it up) it usually happens because the person is focussing more on themselves and what they're doing and as a result subconciously blocking out the other players, rather than thinking of themselves as part of a bigger puzzle or one cog within the wheel.

If you are able to isolate what you are playing "with 1 ear" and with the other, really listen to the pulse or groove of the other musicians, chances are your timing will be just fine...and it won't be something you need to worry about or think about it. A good, rock solid groove is a good thing for certain types of music. For others, (i.e. upbeat bluegrass) a little tempo fluctuation or "breathing" is equally good, as it energizes the music.

I can't recall ever hearing a recorded or live version of Orange Blossum Special that didn't speed up a little bit compared to the count-in tempo of the song. Then again, maybe this particular tune is supposed to speed up, because a locomotive is being emulated. Hmmmmm....