As suggested by Calfzilla & Meso-z, after the back is a good time. Because the ab wall is a counter balance for the lower and upper back. The ratio of strength should be on par with the back. In that case might treat the ab's as any other muscle group, like BB'ing rep's of 8 to 12 for example. Or even 3 to 5 reps. All with added weight of course. The ab wall is a powerful unit when trained that way.
With that core (really do not like using that over worked word) strength of ab's and lower back, the squat, DL, all push/pull movement/overhead lifts should improve very well. Good Luck.