[OXO cube]

hi Gielsy

since almost all after market wheels use a grub screw assembley to attach the wheel securely to the axle 
a +/- of probably up to 5mm is acceptable in axle length ( inserts can hide a lot )
i think the vast range of axle lengths available is such to accommodate this 5mm variation 
i usually just measure the width of the body and work from there 
but same as Kevan , i tend to use drill blanks and bespoke cut accordingly for each car 

regarding the 'old' type screw axles , i have found some small variation to track is achievable using the locking nut behind the wheel 

Kev