When using a PCB heatbed, do I need to use an aluminum plate over top, or can I just use a piece of glass?
Just a piece of glass is fine.
Glass is actually better because it will lay flatter.
I use aliminum on top. Better conductivity
There is no reason to use glass if you have aluminum. The whole reason for glass is that it’s flat. Putting aluminum over glass is absolutely not useful. Use an aluminum plate with a heater on it, or use a glass plate, or use glass over aluminum. Any one of those setups is useful and has pros/cons.
@John_Bump , nothing sticks to aluminum. 
Oddly enough, AL is all I’ve ever used (well, with tape over it) and it’s done great for me. But, yeah, trying to print on aluminum sitting on glass is the worst of all worlds.
Flat (within 0.05mm across a 10" x 10") 1/8 aluminum plate, washed with Windex, with 3M blue painters tape on it, bolted to a PCB heater bed has worked great for me. I’ve tried glass and actually prefer the AL with tape. Conducts well and responds quickly to temperature adjustments when I need it to. No warps, no tape peals, and parts pop off the bed easy.
Glass gives a smoother bottom surface and does not bow at all.
I don’t use bare aluminum… I currently run a MK2 pcb heater underneath my 1/4in mic6 aluminum bed with a sheet of PEI on top. Gives absolutely flat prints with excellent heat spread. missing/deleted image from Google+
My 90% finished mendelmax has a mic6 bed. I’m looking forwards to finishing it. My monoprice mini just has a thin sheet of cheap aluminum, and with a layer of tape over it and nothing else, I have yet to have an adhesion failure.