If a significant portion of the leaves are dirty-- to the point that you feel like it might be interfering with photosynthesis-- you should mulch under the plants and hose them off.

Thanks jessica, there was a thick layer of dirt on that was actually pulling the plants down so i did go ahead and follow your advice. All I have for mulch is pine bark mulch, I've held back on using this on the vegetables though, because I heard something about it being acidic and bad for the plants. Is this true?

I'm using pine mulch, and it's fine (I bought it in bags, rather than cutting it up from bark, though). I have it in my bucket tomatoes in substantial quantities to keep the soil from packing together, and they're huge and doing well.