I can tell you as a former travel agent that most agents don't expect to get a tip. Most travel agents now charge a flat fee for their services. They do almost all work on commission which yes, generally is about 10%. However, for the past 10 years, nearly all airlines no longer pay any commission or its very small ( something like 5% of the base fare before the taxes) so chances are, the agent made no money on the flights booked for you.
The bottom line is, they don't expect a tip but especially if they haven't changed you a service fee for planning your trip, it certainly would be appreciated.