I didn't for the 8 or so years I was in academia. Including not disclosing I'm ADHD, officially, although I was open about that one on a personal level. LOL, to the point of giving lectures on it at a few different panels & symposiums.
My personal attitude, growing up with ADHD is that it's a pain in the ass to seek accommodations in advance of need, and that it often creates more problems than it solves until need presents. That it's far far easier to work around the system than within it. So if there is a need? Burn that bridge when we get there!
- All disabilities are covered (in the States) under Medical Withdrawal (MW on transcripts, which are legally not allowed to affect your GPA or Financial Aid Status, unlike a W, or HW: Hardship Withdrawal). I've used those a few times, most of the time it's a smooth & well worn process. I haven't needed accommodations after the fact, but it's also one of the fast ways to snap them into place. Medical Withdrawal clearly shows present need, & the added quarter time allows for processing of paperwork, & schedule building.
- There is a disabilities office at brick & mortar schools that will do a run through with you of services offered (while I don't disclose officially ahead of time, I always check that one out in advance; to get an overview of their local policies, & what is easy-easy for them to do, & to get an idea what I can do better myself). Most professors would generally rather deal with you on a personal level than deal with the disabilities office... So that one I use ruthlessly. The first day of the quarter when syllabi are presented? I generally know that day -from the syllabi & presentation of "Welcome to My World for 3 Months"- how the quarter is going to fly.
One of my hard lines is missing classes. I do. Period. Even in hard sciences, there are work arounds for that (evening lab hours, extra credit, etc. LOL I've gotten faaaaar better grades by being at the proctored stuff after hours getting cozy with the Prof & TAs, than quarters I've missed few classes and haven't attended them!). If a professor is pigheaded insistent that every moment in their presence is gold, and shall not be missed under any circumstance? Pfft. I drop that class and get a new professor for it. That's a hardline personality thing, and the disabilities office won't sway them an inch / it will be a quarter long fight. The professors that the disabilities office loves? Don't even need the intermediary. Kind of a Catch22: if you actually need the office as an intermediary? It's too late. The quarter will suck.
This has turned into a novel, so I'll stop here. There are definite reasons to disclose, as well as reasons to hold off. It really depends on personal circumstance.