Are there ever times when it is better not to hit the Titan in its weak spot? For example, if there is a choice between hitting with a strong color or stunning with a weaker color?
If hitting the weak color will set up a better move or a big cascade, take the weak color and stun. If not, take the damage
I think it depends on what heroes you are using and how likely those heroes are to survive a direct hit. For example, one of my favorite heroes to take with me is Merlin. I want that Titan to blow his special all over himself, so I make combinations outside his weak spot on purpose. I can use the board to my advantage especially if I have a crystal and tiles of a color hero I’m not using. Then I can fling what you might consider ‘garbage’ at the Titan without fear of retribution.
It,s obviously situational, but the most usual answer is to go for the stun, unless you have already blown your Boldtusks/Kirils/Grimms etc and their effects are about to wear off, in that case you might as well do the strong match to get more damage from it while you can, especially if the attack is about to end.
It’s good practice to focus on the stuns while trying to set up any diamonds because the better titans you fight in the future, the more punishing they are if you don’t do that.