How much larger should a plated-through hole be than lead diameter to get good solder wetting?
I don't think you should depend on the holes to locate the part when tolerances are important. I agree with your 10-15 mils as I said earlier here (in metric 0.25 to 0.4mm). 

Instead, make up a jig to hold the parts in the proper location while they are being soldered. It can be fancy for wave soldering or something simple that holds the parts while being hand soldered (perhaps fashioned from the mating part such as the punched chassis). 

Round pins are a particular hassle if you call it too close- at least with square or rectangular pins you can skive off some plating inside the holes at the corners of the pin and still force the pin in. If you insist on the using the holes... um, maybe make the peg round and the hole square.. (call out slot-drilled square holes in the PCB).