If your sending it to a mixing engineer then you'll probably want to take effects off which aren't part of the sound itself. eg. you put a distortion plugin on a bass sound and then a compressor to give it a warmer tone, those sort of plugins you'll want to leave on however say you have a pad sound and you put a reverb on it to sit it in the back of the mix (a mixing decision) take those off and tell the engineer that's how you originally had it/liked it. Though if your sending to a mastering engineer (different to a mixing engineer) you will want to leave every plugin as it is while making sure nothing is going over -0dBFS (every channel not just the master). In regards to your question I mix my music as I go then when I think its finished I'll run it through a mastering type limiter and reference the mix on different monitors and speakers and make adjustments according to how it sounds. Hope that helps, Drew.