Ilford advice notwithstanding, I find a one-minute presoak using a smidge of Photo-Flo to be more or less essential to eliminate airbells when developing FP4 in PMK. It may just be the well water chemistry or my technique, but without this step no amount or style of agitation seemed to dislodge them from sheet film developed in hangers. Adding Photo-Flo to the developer itself reduced, but didn't eliminate, the problem. The presoak probably slows the effective development rate somewhat, but that has been "calibrated out" of the procedure.
For what it is worth, the late and lamented Kodak Plus-X Professional didn't exhibit the airbell problem.