Also, what are the best trinkets I should aim for? :D
If I had one choice I'd take PPP (healer amp) and the second would depend on the encounter. Thok's is insanely over budget on some of the harder fights, so that'd be my second choice to use with PPP. But for spread fights where Thok won't do that well, I'd take Blackfuse followed closely by Nazgrim (multistrike).
Edited: Forgot to mention that choosing Blackfuse over Nazgrim for spread fights will give you a spirit proc that you wouldn't normally have using the other trinkets, which could result in having too much excess mana unless you reforge accordingly (thus wasting the benefit that the Nazgrim throughput proc would provide).
How beneficial is Glyph of Wild growth? I assume its better for 25m than 10m?
Glyph of Wild growth is almost always worth using simply because of the increased mana efficiency of adding an extra target, but no additional mana cost. There are many other things you can do with that extra 2 seconds waiting on the CD with the glyph as well (2 more rejuvs! as an example) and a 5 target WG is not only less mana efficiency, but also less raw healing-per-global than a glyphed WG. Even if the 6th target is overhealing sometimes, maybe some nice/lucky cape procs will make that useful for you. :)
The times glyphing WG could be less than beneficial are when it won't hit more than 5 targets (arena, cmodes, proving grounds, random solo / small group stuff) and extreme 10man situations where your WG is rarely hitting 6 people AND you'd be using it every time it's off CD at 8 seconds if it weren't glyphed. Can't think of many 10man situations in recent tiers where this was the case.
TLDR: WG glyph is good for raiding 99.5% of the time. There are things you can do with those extra globals you get waiting on that more mana efficient / healing-per-global glyphed WG that are almost always better than casting an unglyphed WG every 8 seconds anyway.