I am going to order a set of contrast filters for my Omega D2V. Looking at the 6"x6" filters, I see Ilford has a set of 12 ranging from 00 to 5. Kodak's set is listed as -1 to 5+. Is it reasonable to assume that 00 = -1, and what is up with the 5+?

I am planning on just ordering the Ilford set, but it is a bit more expensive. I will be printing onto various papers, but primarily Ilford at this point.

Example of the Kodak set - http://www.adorama.com/KKPMFS6.html?searchinfo=kodak%20contrast%20filter&item_no=2

More or less, yes. The numbering systems are not identical, and the numbers don't always equate to a specific paper grade number. It's just a number. Print a given negative with a filter from a set or VC filters on different papers and the contrast will be different.