I do think it's worth covering the vent holes to the interior of the toptube, seatstays and fork blades, so you don't have media coming back out at bad times, like while painting, or later getting into bearings. Good thick duct tape like Gorilla is pretty blast-proof as long as you don't linger with the gun pointed right at it. We used wood toothpicks stuck in the little stay/blade vent holes.

We didn't bother keeping the media out of the BB, chainstays, seattube or headtube, cuz those could be cleaned well enough by knocking the bulk out and then giving it a little blast of compressed air. We used the air gun nozzle with a longish tube on the end. Careful with the air though; we had good water/oil traps, but even so a little comes through the line, so avoid surfaces that will get painted, and/or give the frame a solvent wipe before painting. A good idea anyway since even the blast media can have some water or oil in it, either right out of the bag, or via the air compressor.

Make sure you've done all the machining, like reaming/honing for the seatpost and headset, tapping the BB, before blasting. There's almost nothing worse for your cutting tools than abrasive grit from blasting.