While documenting at home, some may have delayed contacting the school until they'd been advised by their attorney, advocate, or legal counsel.
Not doing so may risk a change of focus from the problem you see, to a focus on the parent as a problem. Contacting the school multiple times about the same incident(s) without benefit of advice by your attorney, advocate, or legal counsel may infuse drama or escalate a situation.
While it may be your position that a teacher's or para's actions may have breached an IEP or may have been discriminatory against your son's handicap, your attorney, advocate, or legal counsel may be able to gather additional facts and provide insight as to whether an incident or series of incidents did or did not breach an IEP and/or appear discriminatory against your son's handicap.
A parent wants to be sure that they are not "shooting from the hip". While documenting at home, a parent may wish to review their documentation with their attorney, advocate, or legal counsel and seek their assistance in "choosing your battles".
You may wish to read From Emotions to Advocacy and other advocacy materials and ensure you have prepared sufficiently prior to contacting a teacher/school/district with your concerns.