Krystallas on Protector did some testing on which zones the Titan can be spawned in. (Original post which is subject to purge.)
Zones that spawned a titan:
Zones that did not spawn a titan:
* The shard was a little strange. When I first tried in FBZ, I thought it didn't work and was heading back when I got attacked near the portals in the base itself. However, when I tried the others, I didn't get attacked on the way back through FBZ. Normally, if you try to spawn in a zone that won't, the "ambush flag" stays on until you reach a zone that does. It's possible it spawned farther away and then fell off trying to get to me.
** I seem to remember it used to be possible to spawn Kronos in a PvP zone, but then they changed it. So I only actually tried it in one zone, and when that didn't work, I figured it wouldn't spawn in any of the PvP zones.