They can't fire you for no reason, but I would still be careful. Your employer could start to treat you differently due to disclosure. My first employer, as a general rule, would automatically put anyone who they even SUSPECTED of having a disability in charge of cleaning the lobby area. They weren't allowed to interact with customers beyond directing them to a manager, and didn't have ANY opportunity for advancement.
In Canada, they CAN decline to hire you for AS. We have something called Employment Equity, but we also have something called a Bona Fide Occupational Requirement. If they believe that your disability would impair your ability to do the job's core requirements, then they can legally screen you out of the applicant pool. And for the large majority of jobs, they require "Excellent Interpersonal Skills" as a core requirement. Saying that you have AS on an application is ill-advised. Even in Canada.
Still, it is safer to disclose AS after you've already been hired, but make sure they know you well enough first to avoid ridiculous stereotyping.