If you have a decent base tan, you're probably safe for normal sun exposure on those areas.
BUT..........
If you have areas that normally burn (in my case, nose, tips of ears, cheekbones & around my eyes), then definitely put sunblock on if you're spending more than 20-30 minutes outside, unless you have some other covering (in other words clothing) that covers it.
Also, if you're going to be outside for more than a few hours, put some on. Doesn't hurt, and it'll keep you from burning through your tan. And yes, that is possible. Even people with really dark skin can burn, though it's rare.