I never leave the top off of a bottle. There are multiple reasons for this.
I also tend to try out a new flavor as soon as possible. If I don't like it, I will set it aside for a time to try again later. Maybe my tastes will change. Maybe it gets better with time. Maybe I need to trade it with someone else who does like it and hates what I like.
Only you will know what works for you, but it would be a shame to dump a bottle just because the flavor didn't work for you the first time. (Not to say I've never gotten a flavor that I decided a quick flush was required to save someone else the pain of tasting it, but I mix my own a lot and those taught me what not to do.)