I think it very much depends on where you're starting from and where your route takes you. When we hired from Bridgecraft last year it worked out well because we went south to begin with and came back north on the Wednesday. When we hired from NBD, we stayed north all week and revisited Wroxham on our way to Coltishall, so that was a handy free pumpout as well.
Do you need one midweek? Often yes, in my experience. It very much depends on how many crew you have versus how many toilets.