I'm pretty sure I've seen Steam patch games that Valve themselves didn't make. Also, whenever you see Gabe interviwed about Steam, he talks about it automatically patching all your games to the latest versions as one of Steams prime selling points.
Except for HL2 having a bug that stopped you being able to complete the game under WinXP x64 (they may have fixed that eventually), I've never had any issues with any games under Steam.