Unless you are gong to look at something that is 10,000ft. plus there seem little point of going that high
Doing the first turbine endorsement (Huey) one exercise was attempting to get into vortex ring at 14,000, never did, but had to wear parachute for anything over 5,000. Training in the USN oxygen was required above 10,000 day and 5,000 night. FW instrument flying training was done between 10 and 20,000, never used oxygen as found the mask so uncomfortable, never had a problem, 20YO and fit I guess. Some of the US Army chaps I flew with in country had a fear of anything above 1,500 because of MGB failure worries, took one such lad to 16,000 in a Huey trying to quell his fears. Regularly flew IMC at 10,000 on transits in 212, 412, 76, LSALT in our area being 6,000 though airport at each end being at sea level.