RPF1 Fitment Question

[jonc]

Hi all,

Ive searched prior to asking this question so please dont flame me.

I have 17X8 +45 RPF1 for up front and just ordered 18X9.5 +38 RPF1 for rear.
I have read that some people require a 3mm spacer to clear the rear calipers, where others require no spacer.

I have a 2001 NSX. Stock height.

Should I go ahead and order spacers? Why do some people require spacers where others do not?

Thanks in advance...