When your choices are limited to Ruin I and Ruin II (aka, content where you're synced below the ability to use Ruin III), whether you should use Ruin II boils down to this question:

Are you running out of MP?

If not, if the jog between pulls is enough to make up the difference in cost, then by all means, use Ruin II freely.

There are a lot of longtime raiders who shy away from Ruin II for the simple fact that many of the old Raids were long slogs where MP conservation was a REAL PROBLEM for Summoners. A lot of SMNs who had gotten accustomed to using Ruin II for the freedom of movement suddenly found themselves running dry. These folks tend to admonish SMNs that they see doing the same, with the thought that it's going to come back to bite those newbie SMNs later, just as it did themselves.

Honestly, though, for casual content such as Roulette dungeons and trials, it really doesn't matter which you use.

As for content where Ruin III becomes available... It becomes a very tricky balance to maintain MP output while keeping your MP up. You want to cast Ruin III as much as you can afford to, but figuring out how much you can afford to varies from instance to instance. Certainly, during Dreadwyrm Trance, Ruin III should be your Ruin of choice, but that pretty much goes without saying!