I would imagine Ilford knows better than any of us and has tested this in its lab before making the recommendation.
I personally use Ilford RapidFix at 1+4 and use two bath method both bath being one minute long. (yeah, I don't know any better, I guess?) According to Ralph's data in his book, "Way Beyond...", soaking it any longer will make washing more difficult. Plus, over fixing it without way over doing it is better than under fixing.
I like using two bath method because then the second bath is always near fresh without much contamination where as first bath gets pretty polluted near exhaustion. I'd imagine this is a moot point if you always use fresh fixer as some do.