Read this thread (especially posts by rcw3)
AR accurizing
As per that thread, I use blue loctite, as it does a great job and you can still get the receiver/barrel apart without any trouble you just have to tap the upper off the barrel with a rubber mallet. High temp RTV compound works good as well.
As for the loctite melting, cooking off, boiling or whatever - utter nonsense; loctite doesn't work like that. Heat affects its ability to maintain its shear strength (as in how much force you need to get it off a fastner for instance) - but it doesn't become runny or anything, or otherwise disappear (it turns to a white film) - especially when using it in the confines of a barrel extension, where even if there were lots of heat, it would still continue to be effective in doing its job of making the barrel/upper fit very snug.
And on the subject of excessive heat in the barrel extension......wrong. The heat is generated in front of that. I have taken apart two loctited barrel/receivers now, and on both, the loctite was observed to have remained firmly in place and still retaining its blue color. If it had been excessively heated, it would be whiter (but even then still working in this type of application).
As to whether this is a good idea or not - IMO it is. I once had a barrel which for no reason I could figure, would string shots as it heated up. Torque etc was perfect. I followed the loctite advise, and lo and behold....no more vertical stringing. Hope that all helps.
Edit again - loctite will not "square" your upper - for that you'll need a lapping tool (as sold by Brownells, for instance). But even without doing that, the loctite will relieve any pressure points with the barrel that might be caused as it effectively makes sure the barrel is held by the whole extension and not just the front face of the extension.
NB - before anyone pops along and rants about how you should not put loctite on the barrel nut THREADS.....exactly right. Not what we are talking about. Coat the inside of the receiver where it contacts the barrel, and the barrel extension itself; use moly grease on the threads as per normal. Before you put the barrel nut on, wipe away any loctite that seeps out.