Most of the time I see RF stages coupled with .1u caps, which are
basically transparent at RF. Sometimes I see very small values
used, like 3-10p.

A couple examples are in:

http://www.amqrp.org/kits/38spcl/schematic.html

C17 (near the middle top, connecting U2 to a tank) and then the output
of that tank to U4A (unmarked, but also 5p).

It's clearly important to the circuit (empirically) but I don't understand
what they're doing. I can sort of see C20 (the unmarked second one) acting
as a high-pass RC filter.

--
Ben Jackson AD7GD

http://www.ben.com/