With the release of 4.3 and the nerf to the WG glyph, I have removed it from my active glyphs for general use. This particular fight, however, I'm not 100% sure if it would be an HPS increase or decrease, since it's guaranteed to heal all 6 targets on every cast. I was able to kill the boss without it, but I'm curious if anyone has any actual numbers about HPS with and without the glyph in a scenario with all targets needing the heal.
From my spreadsheet, glyphing Wild Growth is a small ~4% loss of Wild Growth HPS, but gain in total HPS because you will have more mana and GCDs available to cast extra Rejuvenations.
Since for Ultraxion you can safely assume each WG will hit 6 people that need some heal (even in 10 man), you should definitely use the glyph. It's a decent HPS increase - just like in other DS boss fights, esp. both Deathwing fights. Looks like the glyph is still useful after all. Thank raid stacking!
Glyph of WG should be slotted always if you are in a 25m guild. It will almost ALWAYS hit 6 targets, and is almost ALWAYS a HPS increase. You get 2 more seconds to cast 2 more RJ per say, it is more regen and HPS that without.
In a 10m situation it is going to be fight dependent. I did not see you specify either, thus the reason for this post.