The tape serves two purposes, as was explained to me a few years ago. Assuming the rim bed is drilled (as nearly all are), it needs to cover the spoke holes and, secondarily, promote a good seal between the tire and rim. To do that second part it needs to go right up to the edge of the rim bed. You may actually see some folks here talk about "building up" the rim bed to get a better seal.
That said, I have a set of rims that are not UST, but also do not have a drilled rim bed, and they are not taped, and they hold air just fine.