Since it's easy enough for TA's to do a cut-and-paste operation, usually I'll already know what hotel(s) I'm interested in and send an e-mail to my TA asking what Virtuoso has and get a quick e-mail back with the details. Total time on the TA's part is like a minute.

If the Virtuoso rate isn't good, then I'll still have the TA book for me if it's a regular rate that's publicly available - they still get the commission and it costs me nothing. Sometimes helps with the way you're treated at the hotel anyway.

If there's some kind of Internet-only special rate that I have to book directly, I'll be honest with the TA about it. I think most of them (at least ones you want to work with) understand. If the difference is like $10, then I'd just go with the TA, but I've had times when it's been pretty substantial - 50% less on a multi-night stay. No reasonable TA is going to feel like they jerked around in that case if you didn't book with them.

One booking isn't going to make or break a travel agency, and the TA will know you'll have another trip coming up soon enough.

The above is in the case where I asked the TA for minimal work. That's usually my case because I don't get to them until I've already do the research on the Internet and FT. In cases where the TA has expertise I'm drawing on or I am using extra services (e.g. research stuff, contact the hotels, etc.), then I'll almost definitely book with the TA unless the price difference is large. But usually I wouldn't even get to that point because I don't call the TA up blind - I already know what I could get, so if the TA's numbers aren't in the right ballpark, I'll abort long before they've invested too much time.