Allen

Lets just say I've seen more than one Cessna flipped onto its back when tie
downs failed in high winds.. But Pipers seem to stay upright.

I wondered about this.

I found that taxing a low wing is much easier to handle in high winds
situation. Is it because the CG is lower to the ground?

After all, the weight of the fuel is lower to the ground over the
wheels, thus harder to tip over?

A little too simplified. Low wing airplanes get blown over in high
winds as do high wing airplanes. Dihedral, direction and force of the
wing, width of the landing gear all play a role.

When taxiing, holding appropriate aileron and elevator deflection
makes a big difference. Doing it wrong on an extremely windy or gusty
day, combined with misue of the brakes which gets the airplane
rocking, can ruin things for you in almost any light airplane.

All the best,
Rick