with 14mm all around you will reduce understeer. The difference will be noticeable but not overly dramatic. This occurs b/c you are increasing the track width of the front of the vehicle. Essentially, it would have the same or maybe a bit more of an effect as if you'd plus sized the front tires.
Now, if you put 14mm on the rear and none on the front, you would increase understeer dramatically.
To determine what you're comfortable w/ and would like the most, I'd order 2 sets of 14mm spacers w/ the longer wheel bolts. Start off by installing them in the rear and go for a drive. You'll notice much more understeer. Take them off and install them in the front and go for a drive. Here you won't get oversteer, but dramatically reduced understeer (which may feel like oversteer from what your used to). If you really like the feel, then just leave them in the front and put nothing in the rear. Or "neutralize" it some by returning the 2nd set of 14mm spacers and add something like 7mm to the rear (again get correct wheel bolts). To "neutralize" it a fair amount (but not yet back to stock), put the 2nd 14mm set on the rear and call it good
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(most places won't accept spacers back if they've been installed and you can easily tell if they've been installed and driven on that's why I suggest moving them around to try).
Something to consider as well..... different sized spacers require different sized wheel bolts. So, unless you do all your own work or implicitly trust your mechanic, you might find yourself worrying.... "did they tech put the correct wheel bolt length on the correct axle....". If you're ok w/ that then use different sizes. If not, I'd put the same on both axles
Good luck and you'll enjoy the enhanced handling and looks