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

Don't start rolling tubes until you have given it enough time to really hear the sound with the tubes the designer intended. Only then will you have a realistic baseline for comparison if/when you try others.

If you are getting it used, make sure you know what tubes are being provided and whether these are standard design issue or custom tubes that a prior user rolled.

Also, the easiest way to determine if a tube has gone bad is to have a replacement available to see if changing a tube fixes the problem when you hear one.

Now I need to follow my own advice and pick up a spare tube or two for my own tube pre.....

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.