I think it's a good idea, since the map editor already relies on tiles coordinates, the user should find quite easy to use tiles as points of reference for the camera. Sure, some special objects like "camera points" to place on the map would come handy for beginners (and so would waypoints for moving characters, I guess). Personally I won't need this, but as you say, it would be nice further down the road. So yes, the camera placement system you're describing sounds totally fine for now. Decimals would allow a lot of freedom while keeping the settings straighforward.