...when the prepress set up the imposition they will allow for a grind off allowance based upon their finishers requirements, it may well be more than 6mm on the spines, but really this value is irrelevant to you as the artworker...
...the actual grind off will finish at the trim mark of the spine, whatever falls into the bleed area will be cut off...
...glue is then applied for the cover to wrap around, finally the remaining three edges are trimmed off...
...even if the spines bleed contains elements of the opposing pages artwork (when you print to single pages from a facing pages document) it will be cut off, and on top of this, you won't see about 5mm of the pages artwork (due to the covers hinge), unless you, with force, flatten down the publication...
...of course, doing this weakens the publication eventually, but nobody reading a perfect bound publication does this, unless they want to peel out a page carefully...
...in short, facing page set ups are fine for perfect bound work...
...if the print provider uses a CTP system then they can set up the imposition templates to not allow bleed on the spines if they wish (clipping), but this step isn't necessary in my experience...
...if they are manually film planning this publication and don't want artwork/ink in the gutters (spine bleed), then what you are doing might well be helpful. But as I mentioned above, the grind off trims down to the spines edge eventually...
...the spine allowance for the cover requires calculation, this you should be able to get from your print provider, unless they are happy to artwork the cover as a spread themselves, then you don't need to worry about it, other than making sure they know what to print on the spine itself (if anything at all)...
Andrew