Considering how much time it will take to get RRose perma reissue, the very low chance of another event girl stone and how much shards you can gather with promotion gacha, you should use them, unless you're waiting for a very precise girl promotion.
I think you can promote any girl you really like. Unless she is a very bad rainbow (there is some...), she will be useful anyway. On my alt, I promoted Shizu and Easter Cactus Swimsuit. They are not very good girls but they help me a lot on hard missions like Kodaibana and Nidhogg. Nushi, Scene maps and Earthworm are too crazy anyway and you need very precises girls to do the job (or full ampied, full centied lvl100 and max skill teams of rainbows. Haha...).

My criteras for promoting a girl are:

- Do I like this girl?
- Does she looks like a true rainbow? (I mean, a girl with a boring and not impressive skill and plain generic skin or animation will not be good enough to deserve 6 stars).
- How good she is in battle?
- How expensive she is?

If she fits this, I promote. If not, I forget her.
When you promote a girl, you must not regret it after that for any reasons. Choose with your heart, not with your mind.