I doubt there are any hardlines but the difference between 25 and 30 won't be a big issues and even at 30 you can make it cap friendly by having it descend. Over 5 years you have a lot of options. When the wiz, hornets, Lakers, or any other team offer him 35-40+ with 3+1 language the asset goes "Poof"! New cap, old cap etc. the issue is not how you pay your 4th or 5th best player... its if they are good enough to justify the salary.
If the Jazz are at 20 and Walker is at 30 and they can't figure that out... its a failure all the way around. One side likely bears the brunt of the decision. 5/135M where you start it north of 30M and work your way down over 5 years will provide excess value when you really need it.