It's safe for the bike to clamp the seatpost, even on the heaviest bikes. The only thing to potentially be paranoid about is that the seatpost is clamped into the bike securely enough that the bike can't slide down off of it. That can sometimes be an issue, particularly on QR seat collars.
If clamping the post isn't an option for whatever reason, the next best place to clamp the bike from a perspective of minimizing frame damage is the top of the seat tube. However, doing that without fouling decals etc is sometimes a problem. The next place is the top tube, as close to the seat tube junction as possible.
Clamping frame tubes is not necessarily the rookie move that some internet wisdom makes it out to be. It is useful and sometimes necessary when working on various e-bikes, particularly long/cargo ones. However, it has to be done with good judgment.
It's critical to understand that a repair clamp can either be a screw-down style or a cam-over style, and the cam-over type exposes the clamped member to much more force, since the principle they work on is the peak load is a lot higher than the at-rest load. Differentiating between the two styles is important. It's generally pretty hard to hurt bikes with the screw-down style while using remotely good mechanical feel. Accidents can happen fast with the cam type.
The softer/rubbery type repair clamp pads in good, freshly cleaned condition will tend to not be able to scuff paint when clamped only hard enough to hold the bike and keep it from twisting. If it's a harder or slicker pad type, it can be hard to reliably keep the bike from twisting in the stand, which is the typical way to hurt the paint, and in those cases you'll need to use a clean rag to clamp around. When the pads are soft and clean and hold the bike nicely from being able to rotate, it's generally better on the paint to not use the rag.
The thing about clamping frame tubes in general is that when you use a rag, they want to slip all over the place, which is annoying, and when you don't use a rag there's always a level to it where you only hope you have it clamped such that it won't slip and scuff the paint, since it only takes once. Therefore, doing it for no reason is bad.