While you needn't worry, you should reveal facts that you are required to reveal. Graduate school application forms are not easy to obtain (most institutions require online application via an application-handling company), but I could determine something about a few institutions.
If you were applying to the universities of Arkansas, South Carolina State, Hawaii, Alaska, or Tennessee State, there is no applicable question on their graduate application form. Missouri State asks about charges pending, Virginia Tech asks if you are "you currently on court-ordered supervised or unsupervised probation or under the terms of a finding under advisement". North Georgia asks about criminal charges currently pending, and Florida asks about being charged with a violation which if pending could result in probation, community service, a jail sentence, or revocation or suspension of driver's license (explicitly noting that if your record has been expunged, you do not have to answer 'yes'). Indiana is very confusing: it asks "Have you been convicted of a felony or have you engaged in behavior that resulted in injury to person(s) or personal property", but this is immediately followed by their "Felony Statement" that "We require applicants who have been charged with or convicted of a misdemeanor or a felony, or who have engaged in behavior that resulted in mental or physical injury to person(s) or personal property, to disclose this information".
You should start from the premise that nobody actually cares if you get busted for pot, and if they ask any such questions, just say yes and explain (tell only if actually asked). Otherwise, you should discuss the legal arcana with your attorney, to see if you were charged with a felony (probably not), a misdemeanor (maybe not), or some other kind of violation / offense. I think in California, it is a "civil infraction" with no possible jail time. That might be a "violation" or a "charge", so talk to the attorney (or, simply say "yes"). It's vastly easier to just say "yes" and explain. The worst thing you could do is tell an untruth.