I don't have a printer yet but I thought about this and looked it up, and sure enough you can strengthen prints by heating them up to a point a little lower than their melt temperature and holding them there for a while.

Anyone try this? Have you noticed a real tangible difference in strength of the finished parts?

Is this a given or can parts be annealed without distorting? I thought that was the point of keeping them below the melt temperature.

I wonder if it could be prevented by using some kind of fixture to hold it in place?

There are some PLAs out there which are meant to be annealed. Protopasta makes one, it's called high temp PLA or HT PLA. Supposedly it warps less. I think it is a given that it has to warp some, though.

Stefan at CNCkitchen (excellent videos on material property analysis of many filaments if you havent watched them) has some great content on annealing various filaments. I suggest watching it.

I wonder if a hot water bath would work better than an oven?

I'd rather just use a stronger/better filament to begin with, for the most part. ABS, ASA, PETG, PC, Nylon.

I think you can limit the warping by putting plates over the prints to hold them flat, but that requires them to be flat prints to begin with. For anything complex that requires dimensional accuracy, it seems like it would be a real pain. You'd need to really work out the tuning on the dimensions to account for the distortion of the annealing process. Not impossible, but not really worth it unless you're doing some volume on that piece. But then, you'd be better off working up a casting or something if you were doing volume.