ORIGINAL: jester_s1
The muffler is at minimum torque when cold. It will tighten with heat as the aluminum expands as previously mentioned. If you go just a litle past snug when cold, that will leave enough stretch in the bolt to allow for expansion when hot. Locktite is handy to keep the bolt in place, because the oil from the fuel along with the muffler's vibration is bad about rattling it out.
The bolt also grows in length when heated so I don't think it is necessarily tightest when hot on every example. Plus the hard steel of the bolts and aluminum of the engine & muffler behave very differently.