How important to replace replace relay with glow plugs?
 

Should replacement of the relay always accompany replacement of the glow plugs? Under what circumstances would you replace the relay?
Also, is a "reamer" essential to the process?- looks a little pricey!

You should not have to replace the relay unless it goes bad. Search around - you can usually find a good, working used one.
Reaming out the holes is something that should be done if you have the tool. I would not lose any sleep over it if you are not able to ream them out.

I remember reading somewhere, probably here, that you can use a 6mm allen wrench to clean out the glowplug hole.