Coherent Sampling vs Windowing for 50/60 applications
That's coherent sampling.

Assuming this is a AC power application: it's only coherent sampling if you derive the sampling clock directly from the AC frequency using a PLL (Phase Locked Loop) or equivalent.

But does this account for frequency variations?

Depends on how you derive the sampling clock. If you use a PLL it will track the AC frequency, if you use an independent clock, the frequency will indeed drift.

Should I use windowing? What other approaches can be implemented?

If you really just want to measure the RMS of the signal, you don't need coherent sampling. Actually incoherent is better. You just need to make sure you have enough samples.