I'm also experiencing this problem. I have a brand new 2015 MacBook running 10.10.3 and the MS Office 2016 public beta installed.
I've tried deleting the com.apple.iconservices.store file as well as deleting all three cache folders (Library, System/Library and User/Library). Neither action fixed my problem.
The one thing I can add is that I did find the icons are not completely broken on my machine -- they're only broken in Finder views with a text size of 12 or below. If, in column or list view, I increase the text size to 13 or above, my Word and Excel icons appear correctly both as a document preview (with icon preview checked on in the view options) or as a graphic icon (with icon preview unchecked in view options). If, however, the text size shrinks down to 12 or under, it does not matter if icon preview is checked or unchecked -- in both cases I'm left with a blank icon only.