Many years ago, a league officer had to watch the pre bowl. The other team was offered the choice to bowl with that team so both would be bowling on the same conditions. If they chose not to, they had the option to watch. Like someone else mentioned, with modern scoring systems, it's much harder to pull off any monkey business.
Every time I've dealt with pre bowling, the scores were always provided in advance. Unless the score was pretty bad, very few people have the ability to get up on a score, then throw off as JessN16 said. If you're bowling live and you opponent is struggling, how many people go, man I've got this game wrapped up early, let me bag off the rest of the game? It might come to bite you in totals later. I guess there's a slight advantage of bowling a known bad pre bowl score from the beginning, but even so, over the course of 100 of so games during a league, how much could you even bag off of bowling against 1 or 2 bad pre bowl scores? I bet it wouldn't even add up to a pin. True baggers usually bowl 1 complete league averaging well below their ability and maybe sub in several leagues averaging "normal", but staying below the 21 games it takes to establish an average. Averaging 30 or so pins below your average to clean up in handicap tournaments can help a bagger, 1 does not. I'm not sure what real advantage they would gain in txbowler's situation for the current year. Maybe an extra stick (if it's a handicap league) if everything went perfectly, maybe still fit under a cap next year?