Negotiable instruments act,1881

The requirement of giving of notice is mandatory...

 If no notice making a demand for payment is served upon the drawer within 30 days from the date of dishonor of cheque, a complaint is not maintainable unless the complainant satisfies the Court that he had sufficient cause for not making a complaint within such period.

Notice means a notice in writing...

The notice need not necessarily be by registered post only. It can be sent by a telegram, fax or by a letter as well.....!

However it is preferable to send the notice by registered post, as that is clear evidence of service...!