As per CIC website http://www.cic.gc.ca/english/visit/letter.asp, it is not a mandatory thing to have this letter notarized. However, having notarized stamp may only eliminate chances of refusal, as if Immigration Office might refuse if letter is not notarized. Its a grey area and IO has the liberaty to refuse or grant Visitor visa if there is no stamp.
I, personally, have never stamp noterized a visitor letter. However It might also depend for whom you are sending your letter to. Spouse, Parents, Siblings or Relatives. If it is for relative or friend, I will suggest to have your letter notarized.
Regards
Yasir Majid