The short answer to whether that is a good phrase to be using is that it depends on where you are. Outside of the wind tunnel of American politics, "liberal" tends to mean that many liberties are afforded you, with the implication being that you have fewer regulations and rules. But in America, liberalism is not merely a factual description of how encumbered $SOME_ENTITY is, it's also seen as an ideology that stands in opposition to conservatism. So it's a bit overloaded. If you're living in America, you probably haven't been using it wrong, per se.

That said, I have no degree in political science. So, WTF do I know. Here is your grain of salt. You're probably right though that I should not use "fiscally conservative" as the opposite of "fiscally liberal", because they are not really opposites.