Beyond 531 has you work up to your TM each session. This should be 85% of 90% of your EVERYDAY max. So say you squat 100kg on your very worst, flu ridden day, you would take 90% of that (90kg), and then set your training max as 85% of that. Gives you a TM of 76.5kg that you work up to for a rep pr. Then add 4/8kgs per set to that weight (84,88,92,96,100).
If you have set your TM right, you should be good for at least 3 or 4 joker sets, even on a sub par day (whether you actually do them or not).
I think you guys are talking about two different things. You can and should hit joker sets after your regular 5/3/1 sets if you feel up to it. You don't have to, and shouldn't make a jump if you aren't confident you can hit the reps. However, after hitting joker set(s) for triples (on 3s day) or fives (on 5s day), you can continue working up hitting singles if you want.
HGStripe is talking about the "Beyond 5/3/1: Training Maximally" program which is a different animal. But I don't think the interpretation of percentages is right...I read it to mean taking 85% of a comfortable gym max as the training max, not 85% of 90% of that number. It should still leave you with a very manageable training max, something you'd probably be able to hit for 6-8. You'd increase this slowly, no more than every 3 weeks but potentially much less frequently. This is what I've done and it's worked well.
FWIW, I may well have messed up with the 85% of 90% thing, but my current TM is now up around my previous EDM (4 months later), and I'm hitting it for 8/9 reps on my TM set across all four lifts. I don't make the point to argue, it just goes to show that there's no harm in starting light, and getting stronger every time you go in the gym. However it's diced, that's a hell of a strength increase over 4 months.
To be fair, I used a comfortable gym max to base my numbers off of, rather than the true limit of what I could hit in the gym, so my actual numbers fall pretty much in line with your 85% of 90%. I agree that with the Beyond program, it's hard to set the TM too low. You are free to lift as heavy as you want, that's why the jokers are in there.