Steve, it depends on what sort of grain magnifier you have.
Originally Posted by optique
When I moved up to 5x4 I was using a Paterson Focus Finder - it worked fine on 35mm & 6x6.
However on 5x4 there was no grain to see at 10x8 or even 12x16.
However, if you moved the magnifier around the print you can find details/edges/something to use to help you find focus.
After a while of doing this I got really fed up with it, so I crumbled and bought a Peak Focus Finder - and the grain has returned even on 5x4 - but they are expensive.
If you can manage with your standard focus finder and edge/detail/something to focus on then stick with it and save your money.
It might be worth keeping an eye on E-Bay - they do occasionally show up.