I voted that horses should be tied solidly, but I also wanted to clarify that the individual situation may call for something different.
First and foremost, before a horse is tied solidly, they need to be taught how to tie properly. They should give to pressure on the halter in every direction before you even think of tying them to anything. After that, you should be able to tie them to just about anything without having to worry about them having a freak-out moment where they set back against the halter.
I have one of the support beams of my barn (it's a 6x6 buried in concrete at the bottom and bolted to the rafters at the top) that I use to tie solidly. I've got part of an old lariat looped around it twice that I tie to. That will hold up to any size horse, no matter how determined. How do I know? I've had them try to break free.
Once you know your horse will tie solidly, you don't have to worry about them breaking free, no matter where you are. I like having the knowledge that I can leave any of my horses tied to the side of the trailer at a rodeo while I go get something to eat and they'll still be standing there when I get back. I like having the knowledge that I can tie my horse to the corral at the loading pens while I leave for 2-3 hours and go haul cattle to a pasture.
My only exemption to tying solid is when you're talking bridle reins. I don't carry a halter with me and I don't leave a halter on my horses when I'm riding. So, if I have an instance where I need to tie my horse and leave him, I have to do it with the bridle reins. I will not
tie solidly then. I'll take a loop around the fence where I'm leaving them. That will give them enough resistance for a good horse to stay where he's at, but if something bad happens, he can apply a little pressure and get away without injuring his mouth or breaking my tack.
BUT, once a horse learns to tie solidly, he should stand like this, completely relaxed, no matter if you leave him for 5 minutes or 5 hours