As with all things cast, it depends. If you're looking for top accuracy and want to prevent leading for high pressure loads, then use the gas check. For mild plinking loads, it should be fine without. I've always used gas checks on gas check design molds, but that's because I've had a suitable plane base mold to use.
For bullets that have several driving bands, they should stabilize w/o the gas check. But a bullet with short driving bands w/o the gas check it might cause accuracy problems.
