If you are doing quick instrument changes then sitting down is easier. I play the melodeon so it is not so important but for the guy who plays the string things sitting down means that he can play without straps.
Another thought, if you are playing standing up then you need DI or radio mikes. If you don't have them then you are tied to a static mike and so might as well sit down.
I agree about reading the dots. Nothing makes a band more leaden than staring at music stands.
However, at the end of the day (spot the cliche), there is no definitive answer - just have to make a decision bearing in mind all the valid points given previously in the thread!