I have a 91 5.0 f150, and the idle gets pretty rough when it goes down to 600 to 700 rpms. any more and it runs smoothly. I think it started after we took it in for a tune up about a year ago or more or something. Is it normal for shops to set the idle really low? I've heard that they set it to factory specs, but on older engines it causes problems because they are more worn and cannot always keep up with the stock factory specs.

 

The idle is not normally adjusted unless some one has changed it out of spec.
The idle air contol (IAC) controls the airflow for idle.
The actual idle speed is set by the computer program using the IAC to find and keep it within a narrow range.
Your IAC may be dirty and not able to adjust.
The manuel throttle blade position can be set by starting the engine , removing the IAC plug and setting the idle to about 550 rpm then reconnect the IAC plug.
However, with a prpblem like you now have, you can't set it with a rough running engine.
What I would do is clean the IAC and the throttle body with TB cleaner.
Then the EGR needs to be checked for operation and cleaning.
Leaking EGR will make the engine rough as well.
Bottom line is you need this engine gone over by some one who understands, due to it's age.