here is my experience on cams in general. The towers are put on every engine. I am hot positive of the procedure Nissan used to do the install. Typically a part like that is installed and a line hone sent down through the parts to make them correct, but these parts are a little different. You dont have any way to compensate for the part being out of alignment, once the hole is drilled and honed, that is it for that part.
All of the heads are machined to a specific tolerance for a reason. The towers are indexed with locating dowels. I say go for it, if you are really concerned about it then get a dial indicator and you can find out very quickly if the cam bearings are making the cam run out.
The machine shop can check this for you if you want to. What is done is this. You locate the head on a flat surface (granite table) and take heigth readings of each tower bearing. Slide the cam off center of the bearing by say .250 thousands 1/4" then use the dial indicator mounted on a good stand and see what kind of debth differences ther are. I would say if there are more then say .002 then you need to fix that, that is the part that can be time consuming but doable.
I would be willing to bet tho that nissan kept a very good tolerance on all of the parts.
I had my towers removed and replaced with the oiler type tower configuration for the oil bar and I have had no problems with it yet and I have put some high rpm's on my engine from day one.
