non level steerer tube top???

Hi,
I've just bought a second hand Marzocchi MX Comp ETA for my XC bike (off the
net!), fork is in excellent condition, looks like it's never been used. Only
1 problem the Steerer tube has been cut at an angle, the cut is neat &
smooth but it goes down a few degrees from front to back it's not \ but it
isn't - either, If I flatten this with a file it'l be about 3 mil to short
(Ie about 12-15 mil below the top of my steerer) Now every text I've ever
read on fitting a new threadless fork goes to great pains to say it must be
straight, but non of them explain why or what will happen if it isn't. At
the moment I'm feeling better leaving it off level than making it too short,
an ideas from the experts??
I wish people would either learn to cut straight or use a tube cutter :-\

MickyD