You're right. AIM 1-1-20(a)(8) tells us that we should not use GPS altitude.
When WAAS is in common use this will probably change.

I was not using the GPS for altitude. Always use the altimeter
The altimeter was showing 7000.
ATC was showing 6700
Gps was 7060. However when I changed altitude the gps stayed frozen at 7060.
Hank