Conventional wisdom says to train abs last because they're stabilizers and we don't want our stabilizers to be fatigued during other exercises. There's some truth to that. But for a lot of lifters, training abs first just works better.

Training abs first works especially well if you don't really enjoy hitting abs, but you do like to train everything else. Why? Because once you're finished training the good stuff and just have abs left, you've got nothing to look forward to. Plus, you're tired from the workout and may not have as much time left as you'd like. So what happens? You either half-ass it through the ab work, cut it short, or just skip it completely.

Start training abs first while you're fresh so you can go hard on them and still have something to look forward to afterward. Eventually, instead of not wanting to hit abs, you'll get to the point where you like doing them. Sort of like the kid who holds his nose and begrudgingly eats his veggies just to get to the ice cream.

Chances are your performance won't suffer at all. You can still bench heavy with tired abs. Abs recover super fast, and the more trained they are the quicker they'll recover. Even when moving on to legs after training abs, you spend enough time warming up for squats that your abs will no longer feel fatigued. Besides, squats and deads hit mostly the posterior core, not the "six pack" muscles.

If you have a super heavy leg day scheduled then don't train abs that day. But that's probably once a week, so you should still be able to train abs with a pretty high frequency, two to four times a week. Plus, you'll get better visible results doing it this way, so it's a win-win situation.