Waitlist at top program with another offer

Essentially the title: I'm on the waitlist at my top-choice program with an offer at another program. From past posts it seems like in this situation some people receive offers off the waitlist after having already accepted post-April 15 and end up backing out of acceptances. It's important to me not to do that as I want to maintain a good relationship with my would-be advisor at the school where I have an open offer. Would it be a faux pas to ask the school where I have an offer whether I could let them know by, say, April 18? I just feel very stressed at the possibility of accepting there shortly before getting an offer at my top choice and being in a tight spot due to the common deadline. Any advice is appreciated!!