VFR on-top is acceptable in US for PPL holders.
As far as I know, no legal requirement to navigate using visual references VFR. Just need to fulfill visibility and cloud clearance limits. There are times, e.g. transitioning over large pools of water, when you simply can't do anything but using VOR/GPS or just dead reckoning until landfall. Personally I wouldn't base my navigation on VOR/GPS when a chart can be used. I see it as an art making my way forward with just a chart & compass. GPS and NAVAIDs are a great back-up, last resort, tools. But for my own pleasure if I get lost I would try to find myself on the chart first. That's me, other's opinions are respected.