What's the point in saving them? You could be rolling units that could potentially be helping your team right now, maybe even saving you some fodder that you might otherwise waste on lower tier units. The idea behind chances is you can never "expect" anything. The idiocy prevalent in a lot of the people that play this game in their thinking related to the gacha is ridiculous.

The sole valid reason you could ever give for waiting on SP summons is you particularly need a certain class of silver for AW that you expect more to get added to the game in the near future. (Nenya and Mischa specifically, new story maps, etc.) Otherwise you're just being a superstitious fool.