I'd say that setting it up ahead of time to practice is always good. What you may find is that monitors set up in one venue may not feed back, but do in another venue.
Other variables - angle of the mic towards the monitor, type of pattern (cardioid vs. hypercardioid) and how close the singer is to the mic.
Dave Rat has a great video showing how aiming your mics can help place the monitors in the null points of their coverage, which helps.
For my own band, I tell our female singers to be no more than 1 inch from their mic when singing - basically the width of their hand. When they're close on mic, it's a lot easier to have a better gain structure. If you have singers that are 6-12" off the mic, you've got to crank the gain to get any levels, and then you'll have to hack the EQ because now the mic is picking up everything.
So I'd work on mic/monitor placement and working with singers first, and you'll find that feedback becomes much less of an issue.
FYI - haven't played with the RTA yet, but should help to show offending frequencies better than guessing by ear.