You may be better off cutting the steer to length. Here's why. There's always a bit of compressive loading as you ride on the headset. This pushes parts together and creates looseness elsewhere in the headset. It isn't a problem when stuff can't really rotate against each other, but when you have a big unkeyed spacer in there, you just increase the likelihood that your headset locknut will come loose or that your headset top cup will readjust slightly. Neither one is particularly good. Threaded headsets like to deteriorate really fast if they're not adjusted precisely right and you're stacking the deck against you. It's not a big deal cutting the 5 mm -- I prefer to use the Park steer trimming tool and a hacksaw, but any way you want to cut it that gives you a square end will work fine (if it isn't square, the locknut will put pressure at an angle on the top bearings and the headset will bind). Any shop should do it for a nominal fee or for free if they like you.