In general, I tend to spend for 'rebate' events.
Refinement rebate is my all-time favourite. I tend to stock up on medium refines from discounted events like Sage Treasure, or the last Daily Special event that we had. Time-Limited Ninja Recruitment has also been great, it gives quite a lot of medium refines for free, provided that it offers that as a reward, that is.
Summon rebate is also pretty sweet, the summon runes give you quite the boost in stats and power.
I'm currently waiting for the rebate on cave keys. I haven't touched any of the keys I've been able to claim from events for this reason.

Honestly, I probably wouldn't have discovered it myself if Dog of Hell wasn't so * elusive, and I really needed it to make one of my lineups better. ;P

Most people tend to use their summon coins to purchase particular summons, but they're honestly far better off waiting patiently and hoping for good luck from their pulls. Personally, I'm only spending mine on attaining Kirin, now that all my runes are at a high enough level that the runes you can get from redemption merchant don't really help me anymore.