It seems most people in here say don't mess with it and keep training. On one hand you will eventually hit a point where your TM is at or above your max. On the other hand you might not enjoy 20+ rep PR sets so I can see why you're questioning this.
I didn't max on Deadlift before starting my program and I underestimated my max. I maxed yesterday and will be adjusting my TM for my next round.
I don't think you should do this frequently but if you're just starting (like me) then you could adjust it. I don't feel like that's much different than setting maxes before starting but I'm only 6 weeks into 5/3/1. For me this is a one time tweak because I want to lift heavier weights for less reps.
It also won't hurt you to wait. Start too light, remember? You'll be adding 10 lbs to your TM every 3-4 weeks so in 6 months your TM will be about 100lbs heavier. I'm sure your PR sets will stop being marathons by then.