I have the AC68p with the 1 GHz processor so I assume this runs hotter than the 800Mhz AC68u, mine has been at 82C since the day I fired it up and it has been rock stable. I quit worrying about temps, if it comes to the point I need to cut a hole in the router and attach a fan for stability I'd just RMA it. You just shouldn't have to go to those lengths.
Pretty much agree with this. Router design should be such that under normal usage circumstances no overheating occurs. If that isn't the case, the router (or other piece of electronics) is not likely to last very long.