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.....

Whadja get? Is it an autobias amp or one you would have to bias yourself? If you have to do it yourself, you will NEED a multi-meter at some point, and/or a friend to guide you. If it is fully autobiased and the amp doesn't 'eat' tubes, what you really need is time to listen :^)

Extra tubes are OK, but it may be premature. You are better off learning the foibles of the amp with regard to your speakers, your upstream components, and tubes. That takes time, but I expect you won't be overburdened.

Tvad and Mapman are right: have at least one of each type of tube your amp uses available to swap out (in sequence) the amp's tubes in the case of failure. by far most problems you're likely to encounter will be tube failures. you're almost guaranteed to need to replace the odd tube now and then, so you may as well have the spares so you can avoid unnecessary 'down time'.