I find that in most cases it doesn't matter fi you have an ugly pitch, but if the wind should pick up in the middle of the night you may have to get up and readjust, especially with silnylon non-caternary tarps.
I know I have very quickly just setup a hasty flying diamond or a-frame in the dark because I was just too tired to mess with my shelter and just wanted to sleep.
This has worked out for me most of the time, but wind did pick up more than once and had to get up to tweak things.
For an a-frame pitch with a silnylon rect tarp:
1)A tight ridge. Re-tighten just before you retire.
2) Corners tight, but not as tight as the ridge.
3) Side ties should be just barely tight enough to take up any slack.
Caternary tarps, especially cuben or spinnaker ones, are great for a-frame pitches, but are a one-trick pony. It is a very good one-trick though:-)