What GregY said + 1. I would say the advantages of using developer always within the manufacturer's recommended limits are (1) a fresh crisp image; (2) repeatability.
If your reason for using beyond that limit is budget, you might consider a home-brew, perhaps? Or consider buying 5 litres MG developer at a time, which is much cheaper. Then on opening, re-bottle it into smaller amounts, with protective gas. For example, I bought a set of 500 ml and 20ml glass bottles. I divide 5 litres into 8 x 500 ml bottles and 5 x 200 ml bottles. When the small bottles are all used, I open another 500 ml bottle and sub-divide that. I got the tip from a long thread about protective gas on the FADU forum
here. I am still working my way through the first 5 litres, but so far the bottles have kept perfectly.