Some excellent points made thus far. If you are concerned about maintaining the best score possible I suggest getting a Trans Union Canada report. It will show you, as an individual the top 5 reasons your score is what it is, and the top 5 ways you can improve it.
Realtor1 mentioned you shouldn't owe more than 80% of the limit of any revolving credit, I would say you should never owe more than 50% for any length of time.
Also, there is not a huge difference between having a score of say 810 and 690. As long as you are above 680-ish you should have no problems getting approved and qualifying for a free down payment mortgage if neccessary.
Like many have said, the worst thing you can do is miss payments. Even a couple days late CAN hurt your score, depends on the credit issuer.
As far as the Scotia moneyback card, I'm assuming you mean the Momentum Infinite card. You get the biggest bang for your buck if you purchase ALL gorceries, gas, drugs (pharmacy, not street
) and have recurring bills paid using the VISA card. You get 4% on the gas and groceries, 2% on drug store and recurring bills and 1% on everything else. The annual fee is $99, so spending $207 a month on gas alone will give you $99 back each year. The interest rate is 20% though so you don't want to carry a balance on it, in fact all the rewards type cards should not be used to carry a balance.