Have you done a test to determine your clearing time? With T-Max, some recommend a total fixing time = three times the clearing time.
Personally, I've always used a total fixing time = twice the clearing time.
This is good advice. The book "
Monochrome Darkroom Practice" by Jack Coote suggest that 3 times the clearing time is usually safer with films which contain silver iodide, as it is the least soluble of silver-halides. He mentions Ilford HP5 as one such film.
I don't think you would go far wrong by following the manufacturers advice.
Back to the original question of this thread, I still don't see how using distilled water for film processing would affect graininess.