I tend to save my grinders for stuff that has good stats and intend to use it. And since the weapons that grind to 9 (like mechguns) are quick to max out, they are tended to first. Then there are certain non-red box weapons that not only have good stats but also a kick-arse special. My RAmar has a Geist Laser with a mix of stats with 25% to hit that he uses for filling up TP. In my pre-Frozen Shooter days, when I felt like using it for more than its special (sometimes it is damn inconvenient to switch weapons) it doubled as his rifle of choice. Didn't have enough grinders to max it out, but the extra grinds helped.
Demon weapons and Hell weapons (for pre-Ultimate runs) do well as double service weapons. When the special isn't hitting, you can just blast them down. And if you have hit it already with the demon attack, maybe you'll kill it in the next hit.
Other than that, it's a good idea to save grinders for the good stuff (i.e. good stats with hit, good rare weapon, etc.). And if you need to stockpile grinders, go kill El Rappies in Fake in Yellow with a Force! Not only will you get mats and XP in the process, but it's wildly addicting!