I'll speed the chorus up a few bpm sometimes and do other minor changes where needed but I think people need to be very careful with tempo changes. A lot of metal I'm hearing now is so badly written. I'm hearing tempo change after tempo change where the band has tried to fuse together two horrible picking riffs (probably peddling on the low string and harmonising in 3rds

uke

and it just doesn't work - it never will. Transitions are so crucial in any arrangement and tempo changes need to be navigated with great care - only execute if it enhances.
I picked up Opeth's Blackwater Park recently and that album has a lot of good tempo changes (and a few dodgy ones here and there, there's a lot of cheese in that album). They mainly act to enhance the feeling in the music - it sometimes takes your mind away, sometimes goes into something heavy and crushing or something brooding - whatever enhances the listening experience. Their new album, on the other hand, is littered with dodgy transitions and it sounds quite amateurish in places.
From a songwriting point of view I'd say tempo change if it feels right. If you're writing metalcore with aforementioned picking, harmonised riffs - go ahead and tempo change every 8 bars, it all sounds like shit anyway. 2 albums that utulise tempo changes perfectly, for me, are Crack the Skye (Mastodon) and Dark Side of the Moon (Pink Floyd). They just happen to be my 2 favourite albums but its just the fact that everything they do - tempo changes included - enhances the feel/vibe/energy and musical flow.
From a production point of view, if the song needs a little bump in the chorus or a little extra room in the middle 8 - do whatever's necessary, 4 or 5 bpm increase or decrease will appear seemless but will definitely affect the listening experience.