Do lots of siding with my pop's business, and mostly on houses that aren't new construction.
If the top is level, you can use a "false" bottom piece to straighten the wall. You can also shim very small amounts on each siding row, eventually work out a huge amount by the time you hit the top and never notice looking at it. Be careful though how you do it, if you get too far off on a single wall, makes it really hard to come out even on the other walls and have your siding lines match up. A couple measurements and could have a lot more info for ya. Depending on the amount you need to make up and the size of the wall, you might be able to do something with it.