In my opinion it's only important as you want it to be. Bike fitting primarily is concerned with just how you fit while in the riding position and pedaling. So if a smaller frame with a longer seat post gets you there, that is good too.

What kind of bikes are you riding or looking at? Road, Hybrid, mountain, cruiser, etc....?

I generally only take notice of old bikes. But many frames I see today have sloping top tubes in every type bike. Makes them more uni-sex and certainly helps those with shorter inseams and normal to longer trunk lengths get on a frame with a more appropriate top tube length.