The problem that i see with installing them straight in, is that you have very little room for error. As you can see from the red lines, a very slight angle would allow exhaust to pressurize your engine versus vaccum.
Look at teh bottom of image. Installing at an angle, your room for error is greater, so it will be harder for air flow to pressurize.
I always cut pipe at angle and install at angle. And error to the side toward the engine, which means i would rather have more angle then less.
Now to really confuse things, if you have a chock point in the header/exhaust design, you could install a straight pipe, as you would then be working on venturi principle of increase velocity decrease pressure, just like a good old fasion carburator.
But I digress.