Hi Bill,
Here is my opinion about the subject.
Since it's sometime harder to update ERP because of all code you put over it, I tend to update only for those reasons:
1. Something is broken or buggy and as specific CU or Hotfix is supposed to fix it, give it.
2. Security fix, if there is a known security issue, it's a good idea to fix it.
3. A new feature that could help your users to be more efficients in their jobs.
I know that some people tend to apply CUs and FPs when they are released, that way you may fix issues before problems happen to you and it keep you system more secure. As far as I know, Microsoft will not support you if you are too far behind in update... maybe it depend of the contract you have with them.
But I wouldn't install all separate hotfixes, they usually fixes bugs and if you don't have it, it could causes other problems.
For what it's available on version 9, since I'm only in version 8, I could not really help on that but you should be able to find the information on the Microsoft site.
Hope that it will help you to take your decisions on that.
Have a nice day,
Alex.