It'll be fine. The optical cable will carry 5.1 sound with no prob lem. Used to be that you'd send video and audio over two separate cables. The optical (digital) was the best you could get. With HDMI,it's sort of like the optical cable has been rolled in with the video cable. Obviously the technology is different, but I don't think you'd notice a difference between optical and HDMI on a 5.1 system with the same equipment.
At my house we run the Wii through the TV and then the tv audio out over Fiber Optic to the receiver. Works just fine as most Wii games we play are PLII audio encoded.