Mis-matched wheels on a brand new bike... nope.

It's not a bad idea to have a spare set for whatever reason, but not for an upgrade, just for a backup.
If it were my money, I'd spend it on accessories or clothes.

No - SHOES. A new bike with new pedals means you should have new shoes... the ones you always wanted, but were more than you wanted to spend. The ones with the quantum buckle adjuster and Tuscan heel retention system or something.