I read on a bike website recently that "all the best riders know slack seat angles are the way to go" or some such (I am paraphrasing). By slack it appeared that the author meant 72 degrees or so (so 1 degree slacker than a boring old NORBA racer from 1995/80%+ of all the XC or road bikes ever made)
Folks, let's get in the way-back machine and remember some high school trigonometry.
-Assume you've got kind of average length legs and your saddle is at 75 cm (that's a pretty normal average for a 6' tall male cyclist and not all that much higher than many average 5'7" women either as they often have longer legs).
-At a 73 degree seat tube angle, your saddle is 21.9cm behind your bottom bracket and 71.7cm above it (we'll assume there isn't any suspension sagging or anything to confuse us). SOHCAHTOA, baby!
-Now we'll do 72 degrees: 23.2cm back, 71.3cm up. We've moved back 13mm, or about half an inch for those of you troglodytes still using SAE.
Now, don't get me wrong. Moving back 13mm will make a difference in how your bike rides - but it's a difference you could accomplish any number of ways without needing to change the actual seat tube angle on the frame. Most saddles have 40-50mm of total adjustment so you can go forward 20mm or back 20mm - a degree and a half each way. You can put a setback post (up to 25mm is commonly available) and go back even further if you want to.
Of course, where you sit also affects your reach to the bars, weight balance, etc, which is why it's important to remember to design with front center if you're doing a frame design from scratch. If you're just trying to fit yourself to an existing frame and you're worried that the seat tube angle isn't as slack as you need it to be - just push your saddle back on the rails and you're all set.
But that's not the important part, really. Here's the real kicker - when you're riding fast, rough terrain (whether descending or on rolling/flat terrain) YOU AREN'T SITTING DOWN. No matter WHAT seat tube angle your bike has, it's irrelevant because your weight is on the pedals and the handlebars (hopefully not too much on the bars!) The seat tube angle could be 45 degrees for all it matters when you're out of the saddle, and 95% of the time when the handling and weight distribution of the bike matters the most, you're out of the saddle.
So does seat tube angle matter? Yes. But it's pretty far down the list of important things when you're designing a bike.