Don,
Underfixed film often develops stains, which cannot be fixed out later. Underfixing can also result in the loss of image silver, as it degrades when exposed to contaminants. That and the danger of getting flakes of sulfur or silver on your film should be enough to warrant careful monitoring of fixer capacities, activity and shelf-life (especially with fixer stored for a longer time) in order to avoid underfixing in the first place.
Alessandro,
Fixer for film remains active enough to do a thorough job of fixation until it reaches a dissolved silver content of 8-10 g/liter. That translates to 24 8x10 sheets or 24 rolls of film per liter. If you had 24 36-exposure rolls to develop, you could mix up a liter of fixer and use it for the entire batch. That would be most economical. So, even saving up a few rolls and fixing one batch would be lots more economical than using your fixer one-shot. 300 ml is 30% of a liter - 30% of 24 rolls is seven rolls and a bit more. You could cut your fixer costs by seven-fold just by saving up seven rolls of film, then mixing 300ml of fresh fixer to use in a session for all seven rolls. Of course, you'd have to keep an eye on fixing time, which increases as the fixer is used, or, like I do, simply fix for 4x the clearing time to begin with for the entire batch. Heck, you could be conservative and just do four or five rolls per 300ml batch of fixer and still cut your fixer costs significantly.
That said, using fixer one-shot does guarantee adequate fixation; it's just a waste of chemicals and cash. From your posts, I surmise that you aren't interested in saving money and that it's worth spending five or seven times what is needed in order to have peace of mind that your film is well-fixed. That's fine... but please realize that your recommendation to others to adopt a one-shot fixation regime for film is not scientifically sound.
One thing that I've done in the past to not waste fixer when I was processing very little film and at greater intervals, was to use Ilford Rapid fixer one-batch or one-shot (depending on the amount of film I had to process) at the 1+9 dilution instead of the stronger 1+4 dilution. I checked this practice with the Ilford technical representative, who confirmed that, yes fixation will be fine as long as the fixing time were adjusted accordingly (clearing-time test) and the capacity was not exceeded (which would not happen one-shot), since the fixer would exhaust more rapidly fixing film at the higher dilution (not due to by-product build-up, but rather using up enough of the active ingredient that the fixer became to weak to do its job).
Best practices for fixation has been discussed ad nauseum here and shouldn't really be a bone of contention any more. If one wants to optimize expense and adequate fixation, the research is clear: fix to capacity and no more and keep an eye on fixer age and activity so you are not underfixing.
Best,
Doremus