The app doesn't always work for room service. In fact, it never worked for me on my last two cruises. You'll probably want some cash for room service tips. For bars and cafes and the like, there's an 18% tip added automatically and you can optionally add more on the receipt (or in the app, if it works). If you're on a cruise that goes to Hawaii and you plan to have anyone help you with luggage, then you'll want tips for those folks, too.
Envelopes will be provided to you near the end of the cruise. You'll also get a piece of paper that contains names and tips for each member of your team. You can separate that paper into strips, put them into the envelopes, and give them to your team members on the final night or the final morning. You don't have to do this, though. They'll get their tips regardless. If you want to give them extra, then you can add the extra amount at Guest Services or you can put cash into the envelopes with the paper slips.