Spacers are needed with Shimano 10sp cassettes when using hubs that aren't Shimano 10 speed specific.

Sram cassettes (8, 9 or 10sp), shimano 8-9sp cassettes and most off brand 8-10sp cassettes will work on hubs that aren't shimano 10sp specific with out a spacer.

Shimano 10 speed specific hubs (like on the wh-7800 wheelset) will only work with shimano 10 speed cassettes and don't require a spacer.

Confusing? Thank shimano.