Is there a button to press that will lock in a rotate shape function to 90 degrees so the lines easily come out looking straight?

There are 2 main tools to snap objects to 90 degrees.

Firstly, there is the orthogonal lock. This allows you to draw, for example, lines that are 100% horizontal or 100% vertical.

Secondly, there is Snap to Rotation. Using this, you can rotate an object so that it only rotates in multiples of 15 degrees. You can change this multiple in the Options menu -> Snap -> Snap Defaults.

I think the 2nd option may give you what you are specifically asking for.

David Snipp
Stardraw.com Ltd

And there is a third option, which is to type the rotation angle into the Angle (labelled "A°") box of the Position toolbar, e.g 90, 270 etc, and hit Enter.  Note that this option is only available for single selections, i.e. a single line, symbol or group.

Kind regards,
Rob Robinson
Stardraw.com