Don't force it on new users, but ask if they want a tour.
Only use sparingly when new features/buttons/changes are added to the app. Only once per session. Prompt is Dismiss/Tour option.
It's better if your app has an "intuitive" UX. But an un-forced tour can be helpful.
Its better than the sign-in interstitial to announce features.
So many designers seem to think I am opening an app with oceans of time while asking myself the question, "What delights do these wonderful people have for me today?" When in reality I'm opening them to get a specific thing done right now. Which is the moment when I am least receptive to somebody forcing me to look at their product baby photos so I can say, "yes, your baby is beautiful, no really".
The points where I'm actually receptive to a tour are after I've completed my task and when I've just realized that I don't know how to do the thing I want to do.