I don't think its that big of a deal, but I think you've pretty much answered your own question as it varies with people. Some people are really good at managing MP, some aren't. I generally use it whenever its up and I see a healer who could use a boost. Sometimes I'll just tunnel vision though and forget about it until someone dies. Then i usually toss it to healer if they're raising. Either way, I really don't think it matters much since its more like a little cushion to help out.