Is there anything special about rotor bolts? I fitted a new rotor to a new wheel yesterday (6-bolt) and didn't have any spare bolts so borrowed them from another bike.  As I go to buy replacements, is there anything special about such bolts?  I know they're generally Torx button heads, but  I can buy them in steel, titanium or even aluminium.

They're not expensive, and they'd be useful elsewhere on the bike (I'd get stainless) but I prefer socket head screws in general because I don't always have a T25 on me, depending on which tools I'm carrying.

So is there anything special about the material or shape?
The Torx heads are generally lower in profile than equivalent bolts with a hex socket. This solves clearance problems in some fork-wheel-disk setups. 

Torx heads can be made shorter because the star pattern of the hole provides more contact points with a screwing tool, thus allowing reaching the same torque values without risk of stripping the bolt head. An equivalent sexagon head would have to be higher to reach the same torque value.