Do I need a spacer between my cassette and freehub? I am fitting a modern Shimano 10-speed Hyperglide cassette (CS-HG81-10) to an old Shimano 8 speed hub (FH-M565). Fortunately, it is long enough (approx. 35mm) to take 10-speed. 

The hub came with a spacer and I am not sure if I should use it or not. I know that when fitting 9/10 speed to an 11 speed hub you should use a spacer, but I am going up from 8 to 10! 

The spacer measures 1mm. I can fit all sprockets with and without the spacer, however with the spacer the final sprocket will extend slightly over the end of the hub. 

How do I know if I need a spacer or not?
My guess is that the spacer is for fitting 7 speed cassettes (which are narrower than 8-10 speed cassettes).

Try installing the cassette without the spacer, if you can tighten the lock ring up and it isn't bottoming out on the freehub, I think you are good.