I'm just looking at adding an auto top up on my PAYG, but it asks for
which station do I want to use. As I don't have a tube near me, I will
only be using buses. Should I just put a default station e.g. Waterloo?

Your Oyster card needs to be tweaked so that it is recognised as having auto
top-up enabled. In order for this to happen you have to nominate a tube or
DLR station or a tram stop where you will be know you will be using your
Oyster within the next week and it is that which you are being asked for.
When you validate your Oyster on entry to that station your PAYG balance
will be topped up by an amount you nominate when you order auto top-up (you
have to buy some 'manual' top-up at the same time as ordering auto top-up
for some reason) and your Oyster card will be programmed with the auto
top-up instruction. From then on auto top-up will work for you.

The gotcha is, of course, that if you aren't intending to make a tube, DLR
or tram journey using Oyster then you can't get the auto top-up on your
existing card. I had this problem but because I was using the tube now and
again, only with a paper travelcard, I was able to make a short journey
using the Oyster card and get it programmed (and because there was a free
£10 top-up promotion at the time it didn't actually cost me anything).
Another option I know quite a few people use is to touch in at their
nominated station so the card gets programmed and then go straight to the
ticket office and get the journey cancelled.

I seem to remember that TfL belatedly realised this is a bit of a stopper
and that you can now order Oyster cards online with the auto top-up already
enabled. So it might be easiest to just get a new one. Unless of course I
was just dreaming that bit.