I'm using Ipe with mounting clips. When abutting two pieces is it best to scarf 45° or just abut it at 90°?
I would suggest to always (just about) use 90's. You want and need any and all joints to only happen on top of a joist or other structural member and be fully supporting both boards' ends...of course NEVER "float" a joint, not even in plywood or drywall (horrendously wrong practice).
There are however 2 cases that 45's can work fine. Typically for a decorative pattern. One is, when the joint runs inline or parallel with a structural member. Or Two, when the joint occurs on a beam or additional blocking can be installed. Otherwise, the boards can be very twisty if not at least mostly supported and doubled nailed or screwed.