When you build a 1:1 cast sequence macro you do set the timeout to be your weapon speed. This will not affect the macro when you are spamming it, it however is a good idea for any breaks.
The idea is this. If you stop firing and your auto shot goes off you want the macro to reset so that it starts with steady again. This is why you generally put the reset time to your weapon speed. "3" in those rotation macros has absolutely nothing to do with your attack speed. It meant that if you didnt use the macro in 3 seconds (or whatever you set it to), it would reset to just auto shooting. To discuss it in an example, let's assume that you're clearing some trash, and your Tank will be taking aggro from 2 mobs at once. You're using this macro:
#Showtooltip Steady Shot
/castsequence reset=3 Steady Shot, !Auto Shot
/cast [exists,target=pettarget] Kill Command
/script UIErrorsFrame: Clear()
You're happily firing away at the first mob, and it dies. You click your raid frame to take the Tanks next target immediately, and continue firing. Your macro did not have 3 seconds with which to reset, so it's going to fire either SS or Auto, depending on which shot was fired last.
You've achieved the exact same shot rotation as you would have had for a single mob which had twice the Health.
Now you change your macro. You set the reset timer to equal your Auto Shot speed. Same situation, 2 mobs on the tank. First mob dies, you again swap targets quickly using the raid frame. Again, your next shot will be the one next in the rotation. No change.
Now lets say that you're a little slow to swap targets, say 2.2 seconds while your macro is set to 1.9 seconds. So the macro resets and you fire a Steady first, while your last shot could have been either Steady or Auto. You've reset, and have 50% chance to have made a change in your shot rotation.
But my Steady and Auto are pretty much on par for damage. Steady does a little bit more on average, at the cost of some mana, but even over the course of an entire raid this isn't going to make a significant difference in my total damage output.