As simplistic as it may seem on the surface, building cooperative relationships with your various professors is vital. Whether you believe you will need accommodations or are pretty certain that you won't, keeping your professors informed about your handicap is important for several reasons.
First, professors are much more willing to assist you, both in and out of office hours, if you are willing to take that first step. Admitting when we need help is not easy, but it is necessary, especially if you are battling more than the average student.
Second, appearing on short notice and asking for or demanding accommodations is not likely to endear you to your professor. Nor will it increase the likelihood of that professor's cooperation with handicappers who may follow in your footsteps. Making requests well in advance will ensure that professors (and OPHS) have the time to prepare for your accommodation, whether it is a large print exam, extended time or whatever.
Last, even though accommodations are required by law, there are other, less tangible, outcomes of being frank with a professor at the outset of a semester or class. You will eventually need recommendations from professors for employment and/or graduate school. The time to begin cultivating good working relationships and mentorships with professors is now.
Professors, while required by law to accommodate your needs, will likely be more willing to do so if you take the time to introduce yourself, highlight accommodations you feel you may need for that particular class, and keep them updated. No one likes to be bombarded with seemingly impossible requests five minutes or even five hours before an exam.