Did your taxidermist advise you of the measurement or were they taken by you? A 36" cape is a great bull! The neck is the biggest measurement that is going to really dictates the rest of the mount. The other measurements, while somewhat important, are not as critical for a shoulder mount as the neck measurement. Also, make sure to look at several different form companies. McKenzie is a big company but the other one to check out with a lot of different form sizes is Research Mannikins. To answer your question, it very much depends on the form/pose that you are wanting. I just got done doing an elk with a full 90-degree turn with a modified up-turned head on a 34" bull. While fitting that bull on normal, 'semi-sneak' form is no problem, the radical turn-up custom head position really presented serious stretching and work to make it fit properly without having to cut the form down. Every taxidermist will have their own suggestions and ideas, but my personnel opinion is that a rule of thumb is that you never want to 'build down' an animal unless absolutely required. Taking your bull down one size (ie. 36 to 35") is not that big of a problem or noticeable, but working your way down much further will really start to be noticeable. An accomplished taxidermist can also build up a form to accommodate the longer nose to eye/nose to back of head measurements if its a problem or there is too much slack with the cape. Again, its really going to depend on the form you choose. Your taxidermist should be able to give you some good advice as well.
Hope this helps... Joel -BRT