Don't knock it till you try it vs. Don't knock it till you have tried it

The only small difference I can see is that some things can be tried immediately as you experience them (“Don’t knock this tool until you try it” or “... this perfume until you try it”) and other things can only be tried at the end of using them (“Don’t knock the delivery service until you have tried it” or “ ... the recipe until you have tried it.”)

In the first case you are invited to express an opinion at the time you experience the thing. In the second case you are invited to give your opinion at the time when you have tried it.

I would say that "Don't knock it until you try it" is rarely appropriate. There are very few experiences where a comment is valid until after the experience is over so "Don't knock it until you've tried it" makes more sense to me.

There is an alternative which would include adverse comment during the experience which is "Don't knock it before you've tried it" but this is also less common than "Don't knock it until you've tried it" which is the most widely used form.