you definately have to play around. thats the nature of the beast in music. but there is some theory.
i am pretty amature so you should maybe get some more responses from other pros.
i give a boost at round 4.8-5.0khz to add presence. i try to give a boost of about 2db at 5khz then swing my frequency range while listening to the vocals both solo'd and with the backing tracks to get the frequency right. then, after i figure out the frequencey, ill go back and adjust the amount of boost using both solo and with the backing trax again. and lots of time ill go back and tweak the frequency again.
I use a single band eq for that.
That would be mostlikly for the lead vocals.
then with backups you will probably want to take out some of the low end round 200hz that way if you layer it thick you wont get muddyness.
Try taking out some from a bunch of different freq, for each backup or dub track. for example take some from 180hz on the first one then 200hz on another then 210hz for another. and so on.
with that technique you can usually turn get a little more sound into the mix without cluttering the frequency range.
I look at eq'ing in the sense of removing clutter from the mix. most people are into boosting this and boosting that. but dont forget about attenuation. its very important.
Another thing to keep in mind is only make adjustment when your mix NEEDS it. not just cause. you want to make a boost round 4.8khz if your vocals are sounding a bit flat and NeeD more presence. you want to get rid of some from the 200khz range if your track has a really bassy feel and you want to have your vocals to ride at a higher frequency. you might want to even take out way more lows with a low pass filter or somthing. but only if your mix sounds like it needs it.
you can do other things for example if you have a female vocalist, record the leads with flat eq then a complete dub with lows eq'd out, then another complete dub with all the hi's eq'd out. then mix from there. doing things like that is a little more experimental but hella fun. from that sense its all adlib...
sorry im rambling but mixing for me is one of my favorite parts
peace out