FormerlyHemlock
Hero
That's assuming he's an arcane trickster (or caster multi).
Besides, those tactics won't always work. Depending on the terrain (such as a small cavern where the effect will fill the entire area) you might seriously hamper your party. Plus, the enemy could employ similar tactics by utilizing your fog cloud / darkness to deny you sneak attack*.
Those spells are excellent choices for a rogue to have on hand, but I can't imagine using them in every encounter.
EDIT: *wait, no, those are my house rules
Yes, yes, yes, of course there are costs and tradeoffs in casting Fog Cloud/Darkness/etc. All I'm saying is that Greater Invisibility is expensive and overkill for a rogue who wants to hide.