Tipping on alcohol you didn't order because other people order it seems excessive to me.

I pretty much always get iced tea rather than water, which gets billed as a soft drink, and therefor factored into the tip. That said, two things apply for me:
1) better frequency of refills of tea or some other soft drink (or water, at places without free refills) is a big part of the difference between a 20% tip on the total bill and a 15% one
2) being ignored on soft drink/water refills is the sort of thing that gets one dropped to a 10% tip, or in the worst cases, gets a request for the manager so I can make a comment (as I've said in prior tipping threads here, if the service is ever bad enough to tip under 10%, it's bad enough that I'll ask for the manager.)

If I'm at a *really* inexpensive place with table service, I'll always tip a minimal amount ($2, here in the states) jeven if my bill is too low for that to be a 15-20% tip.

As for the original question, I'd certainly tip on an comped item whether it was requested or not, unless it was not really wanted and just accepted out of politeness. If I'm already tipping on the regular cost of a meal and the extra was expensive I might not tip a full 15-20% of the item's price, but just add "a couple of bucks" onto the regular tip.