Im on 5/3/1 also. Its great for squat and dl with minimal assistance but bench and press seems to stall with BBB. After i started doing some chest work on press day it seems to go up again. Going to see after next cycle if ill stall again or keep on rocking :)
I do alot of arm,delt and upper-back assistance work. Basically just to strengthen weak points and to keep joints healthy.
Bench day looks like this:
Db bench 5x10
Db row 5x10
Any row variation 5x10
Lateral raise 5x20
Any rear delt variant 5x20
Db press 5x10
I do these pretty quick and working on the muscle not movement like wendler suggests in book.
Shoulder has been awesome since i started doing the assistance with dumbbells :)
Doing more than 3 assistance exercises per lift makes Jim Wendler cry. Jim Wendler's tears are made of pure fire. You fool, you'll incinerate us all!