What are your plans in T.O?? They have a very good public transport system that will take you pretty much everywhere in the city and downtown. However if your plans include a visit to Niagara then it would be worthwhile hiring a car.
Never had the need (so far) to hire a car, although next time there is a distinct possibility as I plan on going further afield.
One thing to do would be to see if the company you work for has any deals with any car hire companies, these will normally be worldwide (i.e I believe that BA has a discount deal with Avis), alternatively if you work at an airport and have the necessary ID then sometimes on the spot discounts will be given to airport employees.
Sorry this is nothing finite but if I find anything out then I will make another post and let you know.
BTW have you got accommodation sorted at all, by phoning the hotels direct (again if you work at an airport) then they normally will give a fairly decent discount (I stayed at the Bond Place Hotel in Downtown YYZ, cost me about CDN$203 as they had a special deal where you stay for 2 nights and get the 3rd free
www.bondplacehoteltoronto.com) not the fanciest of places but it is downtown and if it is only a roof over your head that you need then it is ideal (PM me if you want more details about what is close by etc etc)
Anyway I hope that this has helped
Rgds
Bluejay