I either prewash all parts for a project or I don't prewash any. All swaps REQUIRE that the fabric is prewashed. It is easier to stick with the prewash because then you don't have to keep track in your stash what is - and isn't washed. Washing takes out the chemicals but it does make the fabric whimpy. To counter that, you can spray it with sizing before pressing.
Batiks, I generally wash because they tend to run a lot. Also, saturated intense colors, I always do a color test to see if they run or not. But it IS personal preference.