The propylene glycol is used as a solvent for concentrated stock solutions in some cases because it does not have the ionizing properties of water and thus prevents aerial oxidization. For long term storage it is not interchangable with water, although PMK developer concentrate has a very long storage life even in partially full containers. Whether you use water or glycol depends on how fast you use up the stock solution. If you do your work in spurts with fairly long periods between, I would say use glycol or mix small enough batches to be used over a short period. Then it will make less difference which solvent you use.
If you are not concerned about children or pets imbibing your stock solution, ethylene glycol can be used. Glycerine is another choice but it is quite a bit more viscous.
If you are trying various developers I would say make 100 ml batches of the stock until you find THE one. Then water is fine.