"Trueing a rim" is a poorly defined service.

There are two things a proper wheelbuilder is trying to accomplish:
1. He obviously wants to eliminate both vertical and horizontal runout.
2. He wants to maintain spoke tensions that are nearly to equal.

When you say that your rim is "in pretty bad condition" it leads me to believe those two goals are likely to be mutually exclusive. If the rim itself is bent it's sometimes possible to pull it straight by using uneven spoke tension but you won't be able to make a good wheel out of it. All that you can accomplish with a bent rim is to delay the time when you start breaking spokes or when the wheel turns itself into a complete potato chip in the middle of a ride.