Mavic wheels all come with a crazy-looking spacer for use with shimano cassettes. Every shimano 10speed cassette i've seen come with a normal, round spacer that looks to be <1mm, and is certainly narrower than the mavic-supplied spacer. When i was running shimano 10speed on aksiums, i used both spacers, and it worked nicely. On most shimano freehubs (Like the tiagra in question), you need only the ultra-thin round spacer. Apparently, some(all?) dura-ace freehbubs are spaced for 10-speed only, which is acually narrower overall than 8- and 9-speed. Hence, the spacer for "lesser" shimano freehubs, not the D/A.
Yes, it is complicated, and probably needlessly so. Hence, some of the arguments made against the "progress" of additional cogs in groups.
-rob