It's all about the chainline, which is arguably the most important thing to get correct on a bicycle drivetrain. Different cranksets may give you a different (and incorrect) chainline using an existing spindle. You can test-fit it and see, but you don't go by whether you have problems or not - you go by whether the chainline measurement is in spec. The good thing about test fitting on your existing spindle is you can measure, and then you'll know how much longer or shorter the new one needs to be.