Usually in OSM there are several different ways to map things, to varying levels of detail. (E.g. As you discovered, you can just tag a road as having sidewalks, or trace out the actual sidewalks themselves.) Generally, as long as the information you're adding is accurate, you're helping. Changing tagging schemes later is a lot easier than re-discovering that information from scratch. If you're really concerned, usually the wiki has good information on what is currently considered best practice.

Incidentally, I think for crossings StreetComplete now only asks about the actual crossing nodes, so no more duplicate quests.