In Myth, any amount of STR is important. I had max True Rage, and I have 1.8k+ STR on my Sieg with my myth gear. I hit 2k+ STR, and my skills were close to triple damage.
I leveled it down, and I have around 1950ish STR with it buffed. My skills do like 222% damage, instead of the 292%.
It's up to you to get it, but since I PvP more than stage, it doesn't matter much to me. For Myth, IMO, it's a must max.