Asking for a taste of a wine if you’re having trouble deciding is perfectly acceptable, but don’t abuse the privilege. Detaining a server to stage a private wine tasting for your table can be a nuisance, especially if you exhibit a sense of entitlement.
If a waiter makes several different recommendations for wine by the glass, don’t automatically ask him to sample you on all of them. Pick the one that sounds most suitable for you and ask if it’s ok to taste it first. Even if you are picky about wine, be respectful of your server’s time. If he offers to taste you on whatever wines you’d like to try, then follow his lead. But don’t just assume he has nothing better to do than stand there watching you evaluate a $9 glass of Pinot Grigio while his station goes down in flames.