Cutting a new rough opening in your ceiling can be a challenge with a reciprocating saw. You can easily end up with a cut that isn't square, especially when you're working in a cramped attic. This thread is more a short tip than a how-to, but I think my information will save DIY enthusiasts some trouble.
As a preliminary step, I build a temporary support using a 2x10 and couple or t-jaks or post jacks (lally columns).
My method for making a square a square cut:
1. Slide a Swanson Wood Magnet (a speed square will work, too) over the joist to mark the cut
2. Position the Wood Magnet to the right side of the mark to use as a straight edge (I like this tool instead of a square because it frees up a hand and doesn't slip).
3. Using a circular saw set to the deepest setting, make a cut using the Wood Magnet or square as a straight edge.
4. Using a reciprocating saw with a short blade, finish the cut using the first cut (kerf) as a guide for your blade.