First Tube Amp: Advice? Keep spare tubes on hand?

No harm making sure you like the amp first before investing in spare tubes. Don't try to "fix" the sound by rolling tubes though if you don't like it to start with.

Once you decide to keep it, get a few spare stock tubes though. Again, you can always roll other tubes later as a "tweak" if you are so inclined...or not.

THANK YOU!
All well reasoned, excellent replies.
When you say decide whether I like the amp first, you mean after the break in period?
The amp is a relative cheapie. I am almost embarrassed to say it. Please remember this is my first foray into tubes, so I decided to just dip a toe rather than jump into the deep end. It is an Onix SP3, which I have read favorable reviews of.
I got a great price on it.
It is a manual bias unit.
I do know my way around a multimeter.

By all means, once you've decided the amp is a keeper, follow Tvad's advice. I also recommend that you keep extra output fuses around; when I blow a tube, I blow a fuse. I can replace my tube locally but not the fuse.

Don't buy tubes yet. As long as you are careful and never mess with connections while the amp is on, you shouldn't be blowing tubes so easily. I haven't blown a tube in years. Yes, I have spares but mainly for fun.