Step 3: Deface national currency. Or use washers.

Picture of Deface national currency. Or use washers.
Drill lots of holes: if you're not using washers, put holes through your spacers & nickels. You want the holes to be marginally larger than the threading you need to accomodate on your monopod. My threading was 1/4" and so I used my 1/4" drill bit and then wriggled the bit around once I'd drilled through.

Where to drill: If you followed my suggestions and have coins with buildings/eagles on the reverse, here's what I've found works.
On not-so-worn nickels, you might still see the architectural detail on the pediment (it's the triangle over the doorway). Center is approximately between the pediment and the top of the doorway. (But see below for drilling nickels off center)
On mint pennies, you can probably see Lincoln in the memorial. Nail him through the stomach. It probably doesn't matter much if you're off center on the pennies. As long as none stick out past the nickels when on the axle, you should be fine.
On the eagle quarter: er. uhm. uhhh. Crotch.

Hints & tips & Learn-From-My-Mistakes: (But you are quite welcome to make your own. And share! I might add to this list.)
-Mark the center of your coins and make a punch mark for easier drilling.
-Unless you've got some awesome ideas about clamping coins up/down/left/right/sideways, it's easier to drill them individually rather than as a stack. (The bit will tend to catch and then spin the coins rather than actually going through them.)
-Clamp the coin on some wood so you can drill through fully.
-I found that some coins will deform is enough clamping pressure is put unevenly on the edge, so I usually place a second coin beside the first and have the clamp hold both down.
-It might be better to have your nickels drilled slightly off center(!) If you tighten the sleeve around two centered nickels, the nickels will continue spinning on a center axle. If they're slightly skewed, the clamping might be enough to have them lock in place on the axle, assuming the holes are close enough to axle size. Drill a couple and play around during final pre-assembly.