Glyphed. You're not going to waste a combust on a MC or small add in intermission of phase 2-3, and you'll pad the meters better if you have a huge 'bustion on the pull and spread that, instead of smaller combustions.
The better ST damage on garrosh himself is worth it over whatever possible cleave you may get on adds.
The only time you unglyph combustion is when you need that little burst every 45 seconds, and that's only been a few encounters this entire expansion (Megeara and Tortos are the only 2 I think I bothered to unglyph).
There's maybe a case for unglyphing combustion on Immersus, but really that fight is so easy it's probably not worth it either. Every other SoO boss always glyphed combustion. There's no turtle-shell mechanic like tortos in SoO.