No you don't.
You don't have to show poll results if you don't want to, and you can show them at any stage later on (as long as the meeting is still in the 'Started' state).
Once you've closed a poll, it's down to you to 'push' the results out. This displays them instantly on all of your participants' devices. If you're using the Polling Projector, then pushing results out will display them on here as well.
If you don't want to show them, don't push them! Press Done to return to the dashboard, or Next to move on to the next poll (if there is one).
If, on the other hand, you're the kind of person who likes to keep your audience in suspense, then by all means press the Push Results button later on.
And finally, if you're using the PowerPoint add-in, then you'll be glad to hear that you've got options here too. Literally... Display Options. Load your Display Options sidebar and you'll see a simple Show Results check box. As a default, the add-in will always display results instantly, as soon as the poll is closed (on the slide, and on participant devices). Deselect this and you can keep them to yourself. Showing them later on isn't quite as simple, but perfectly doable - just search for our article on the Recall analysis slide.