Section 108 the Residential Tenancies Act] (RTA) does not allow a landlord to require them. The same applies to automatic bank account debits or credit cards.
If you're a landlord, be careful not to come across as requiring post-dated cheques (or account/credit debits) and make sure the tenant understands it is just an option if you bring it up. If you have disputes with the tenant later on, it could serve as a mark against you in the eyes of the Landlord and Tenant Board if the tenant makes it look like you required them against their will.
And on the other hand, if you're a tenant and your prospective landlord is being firm on requiring them, it may not be a bad idea to be wary. If they're not familiar with this part of the RTA, where else might they try to pressure you in ways they shouldn't? I'm not saying you definitely shouldn't rent the place over this issue, but it's something to think about.
For lots more information on the RTA, see the [LTB brochure online].