Switch symbol: Which line is normally-closed?
For momentary NC/NO switches, the one shown closed, is the one that should be normally closed. Convention follows common sense.

The given switch though, is a on/on SPDT switch. It isn't really a NC/NO momentary switch, so there is no concept of "normally closed" aside from it being the side you want. If only one throw of a SPDT switch is used, the symbol should match that side, but that's not very necessary in most cases.