You'll have to sim it to get an accurate idea of how you personally should be gemming at your gear level.
When you start collecting enough gear haste will overtake strength. Weapon, tier bonus and talent choice play a large factor in strength vs haste weights. For example haste is usually going to be a little more valuable if you are taking HA instead of SW.
In dungeon heroic gear simc showed strength being a significantly higher contributor to dps but now that I am ~495 ilvl with a 496 shin'ka/gem haste has overtaken strength. I believe my weights are now something like haste = 1.6 and strength = 3.12. So a haste gem = 512 (320*1.6) and a strength gem = 499 (160*3.12).
Edit: Current weights for my gear (496 ilvl, 496 weapon, Sanctified Wrath no 2/4 pc bonus)

In heroic blues it was more like str = 2.8 and haste = 1.2. Strength increases in value but not nearly as quickly as haste for most circumstances. HA will favor haste even further as will 2/4 pc I imagine.