So I'm going to start taming and breeding rexes over the next few weeks and I'm wondering should I wait and tame 150's or tame a minimum level of 135 or so? Going for all hard bosses and hard Ascension
If you don't have boss rex stats to breed with, then yeah anything from around a 140+ could have the potential stats you need to start your spawning program.
Just don't get impatient and settle for one that doesn't have the right stats.