to reiterate, and expanand on some of the thoughts
for high end wireless having the receivers in front of you can be priceless for troubleshooting problems. For consumer level wireless it basicly only becomes helpful to see whether the receiver is receiving signal... (which it might not be if it's cheapo wireless and way far way up in the booth,) Cheaper wireless units, always better to have them as close to the packs as possible, off stage and ideally in line of sight.
all thoughts regarding placing mights the EXACT same way each time by the same people is very important. in addition, each performer should be getting the same mic each time as well. (lable them with masking/board/gaff tape)
Batteries: New ones each performance. and as someone mentioned used the half used ones for future rehearsals, eletric toothbrushes, flashlights, TV remotes, cameras, etc.
I'd stay away from rechargeable unless you keep careful track of how many times they been discharged/recharged, as after a time they will become worse then just reusing half dead ones :-).
CheapBatteries.Com - Batteries - Duracell Alkaline, Procell Alkaline, Eveready Energizer, Rechargeable Ni-MH, Flaslights, CD's, DVD's, iPod Accessories, & Blank Tapes.
sight looks sketchy but they are in fact very well prices, best prices on Duracell Pro Cells i've ever gotten.
BUT most IMPORANTLY HERE IS WHAT YOU NEED when dealing with Wireless:
You will also need an Apple Pie, Caffeinated beverage of your choice, and something that you can bunch with out damaging something to get you through the show.
Also the reason why I use wired mics when ever possible ;-)