Expertise can be treated similarly to Hit when under 4.25% Dodge reduction (2% free from talents) most players however don't see it as mandatory as Hit under 9% is seeing as it's harder to obtain. Humans using Swords/Maces and Orcs using Axes gain an additional 1.25% expertise, so the "cap" is 3%. Also note that Expertise Rating rounds down, partial amounts of Expertise do nothing. The "cap" therefore would be 25 Expertise.
Expertise Rating "requirements":
No racial, No talent: 99 Expertise Rating
Human/Orc racial, No talent: 79 Expertise Rating
No racial, 1/2 talent: 83 Expertise Rating
No racial, 2/2 talent: 68 Expertise Rating
Human/Orc racial, 1/2 talent: 64 Expertise Rating
Human/Orc racial, 2/2 talent: 48 Expertise Rating
* Expertise still doesn't have a definitive cap per se, but due to extensive discussion on the topic it's been accepted that shooting for 6.25% dodge reduction is optimal. If the cap is slightly over 6.25%, it's not worth itemizing to get that tiny bit less dodge when Expertise only comes in chunks of 0.25%. The other hypothesis is that Expertise is impossible to cap completely and functions similarly to the spell hit cap, where there will always be a small percent chance that the boss will dodge, regardless of your Expertise. In either case, shooting for 6.25% dodge reduction brings you as close to the cap without overshooting, if there is a cap.