One thought is this....
Once you choose a mix engineer, send them the original session, as is, and let them check it out. Then explain what tracks you feel might need "cleaning" etc. Let the mix engineer guide you here.
Because, depending on the situation, a given mix engineer might say, "please leave it alone, as is, I want all the bleed and baloney in there", and he (or she) may say this for one or both of two reasons:
1. Because they actually WANT the bleed and noise there so they can use it strategically in the mix
2. Because they don't trust you to clean it properly, they may fear you will make sloppy chops, bad edits, etc and generally make a mess
Or the mix engineer may have a completely different attitude and indeed WANT things "cleaned". BUT, if you have the engineer do the cleaning, they may ultimately charge more because it's their time. The mix engineer may even suggest that YOU do the cleaning in the name of keeping your cost down, at least if they trust you to actually do the cleaning properly.
But ultimately, it's best that you have the mix engineer assess the session FIRST and then let them guide you as to what should be done.
Having said that, any extra "unpleasant" noise that might exist on tracks, unpleasant clicks, pops, hum, hiss, noises that are not a part of the music etc, it's probably safe to say that you can clean that stuff out as long as you are CONFIDENT with your cleaning skills. Sometimes chopping noise out right before or after an important signal can be tricky, you need to slice it just right and apply the "perfect" fade, otherwise it may sound unnatural or goofy etc. If you're nervous about this, just leave everything alone and let the engineer handle it.
Another thought... you can send the engineer both a "cleaned" version of the session and ALSO a "pre-cleaned" version as well. Instruct the engineer to start working with the "cleaned" version, and that if he comes upon any tracks that have issues DUE to the cleaning, he should just import the track in question from the "pre-cleaned" session and go from there. As well, the mix engineer can just choose to start working in the "pre-cleaned" session if he so desires.
As a mix engineer, one thing I'll say, I very much desire to receive sessions / tracks that are very clean, neat, tidy and organized. If you want to piss off a mix engineer (and make your mix bill higher), give the engineer a chaotic session full of dirty, noisy tracks, poorly labeled, regions all over the place, etc. Sometimes it can take a whole day of tedious, boring work just to get a session into a state where precision mixing can even commence, let alone doing the actual mixing. Cleaning and fixing time can wind up being greater than actual mix time. BUT... if your session is generally clean, neat and well organized, you should be ok.