1. march the stud holes.
2 ensure the rim will not chafe the brake caliper or any suspension parts.
3 the tyre does not bottom out into the wheel arches
Nkakamasemts comments are correct but also the circumference of the two wheels, including tyres, should be the same. If there were any significant difference and the spacesaver were put on a driven wheel the diff. would be working overtime and could be damaged. Also the handling would be very poor.