I think all designers have done this at some point or another, and it definitely works. I'm guilty, too. But it's bad practice. Thanks for your disclaimer.

To add: There are so many talented type designers that deserve payment for their work. I understand the allure, and the ease, but as a community, we should never advocate this practice and should do our best to squelch its use. It delegitimizes the work of these designers, even if our intentions are good, to buy it later, and only use it in a mockup.

There are too many whose actions may quickly become habits that slide beyond the measure of good intent. We don't want our clients playing fast and loose with our intellectual property and creative output; we shouldn't be doing the same with our fellow designers, either.

Well, Fontstand is a useful app that has allowed us to trial some good, popular typefaces. Downside: the selection is limited, but it's a start at solving this problem.

Occasionally, for mockups, we'll use a battery of screenshots of web type previews, even in print projects. That's not always helpful (works better for body copy and headlines, than for say, logos).

Otherwise, we have language in our contract for third party expenses. If we think it's the right thing for the project, we go ahead and buy. Our clients are buying our judgment; they tend to trust we're only going to buy what we need.