I always have one on the go. Pop one every time I do something that would have used the last one. Kill an ultra? Pop a coin. Finish a pvp match? Pop a coin. That way I always have one ready and i don't forget to do it before the next boss fight. I don't care how long since the last one I used either. People stress too much about that. I get a reasonable steady drop rate doing this and it doesn't intrude heavily on how I play.