How important is it to wash thorougly the back side of the film?
I develop 13X18 sheets in perspex tubes which I place in Jobo 2500 series
Drums (the large ones) and develop by hand just like rollfilm 120.
I wash using the ilford method X 3, more than adequate as far as the emulsion is cocerned, and then transfer the film to dishes filled with water
for the final wash. I would be oblidged if someone with similar developing
method would comment on that.

I use the print drums. Most of the washing happens in the drums but I dump the film into a tray and run the water for a little while. It just feels safer and isn't a big hardship.

I use a Unicolour roller drum for 4 x 5 and find that washing in a tube does not remove the Antihalation layer properly. In fact, I use a 2x recipe of hypo clear in a tray for two minutes for TMX 100. Strangely, Ilford Delta 100 requires almost 5 minutes and 10 minutes washing to get the entire layer off the obverse side. I do not know why Delta 100 does this but Kodak TMX is fine.

I got the jobo film washer hose accessory and I attach my 2500 tanks with it to the tap and let the water run for five minutes which is supposed to be archival.