You used it well enough. I never found myself accidentally entering a warp, which is good. I used that system once in a test level (it was a very bad test level, don't listen to what bob says about it), and one of the problems was players would always stumble into warps by mistake.

Something extra was thrown in as well, though. If you kept your coin until the very end, beating all 20 levels, you were allowed to play a secret, super hard 21st level. It would be cool if your tests had a similar sort of reward.