No, I don't think that 30 cards is always the optimum deck size. I don't even think that 30 cards is always the best size for rush decks. I think the right deck size is based on a confluence of factors like probabilities, output needed, opponent's deck, etc.
Some reasons you may run a deck with more than 30 cards:
1. You're using a stall or control deck.
2. You don't want to deck-out in case of a stalemate. So you a use 31- or 32-card deck.
3. You need a fast deck with a lot of output. I usually call these kind of decks "tank decks". It's a deck that can blitzkrieg like normal rushes, but it usually has more creatures than a normal rush deck in order to give it extra staying-power and firepower to plow through CC, shields, and healing. It's a little less consistent, but makes up for it by giving you extra output which is handy against certain deck types.
3. You need to balance your quanta. Sometimes when you're balancing quanta, you find that adding a Dragon as the 30th card gives you a nice QI of 5. But playing only 1 Dragon is generally a bad idea because you're balancing your quanta based on 1 expensive creature which you have a slim probability of actually drawing. In the end, you end up being quanta-heavy in almost all your games. Instead, it makes sense to add two 5
creatures instead of a Dragon, giving you 31 cards. This is a better way to balance your quanta usually, imo.
