The memory maxing out of a printer like this is only useful when you plan to frequently use a lot of different fonts. The memory requirements even for high resolution printing is not that much in greyscale. Having stored the fonts in the printer reduces the overhead of tranferring the fonts needed for each single print job, which slows down printing especially when the printer is connected via LocalTalk. With an ethernet connection you will hardly notice the difference. Keep in mind, that you will have to upload the extra fonts to the printer using a Printer Utility software each time you restart* the printer, as the SIMMs are volatile memory. In case the 5M has got a harddisk drive (like some Apple LaserWriters), you might better use this to store your font collection.

*I would not recommend to have a Printer of this age on standby over night, because of the unnecessary power consumption.