It’s a question we often get in class when we reach that part of the course. We demonstrate correct use of masks in every course and you get to practice with them. The answer to the question often will depend on the type of class we’re teaching… 
Workplace First Aid (OFA 1): Yes.
If you are working as a First Aid attendant and a worker needs CPR you are obliged to do it. Current practice requires both compressions and breaths and WCB expect you to use a mask. They are provided in class and everyone practices using one.
Other training courses, especially Child Care First Aid/Infant CPR: It depends….
It would be nice if you had a mask to use, but if a child in your care (or your own child) needs CPR, will you go hunting for the mask and spend the time it takes to use it correctly?
It’s a judgement call and students in class often indicate they would not spend the time looking for a mask and trying to use it. Nonetheless masks are provided in every class and students practice both with and without masks so they can make an informed decision. We also sell the masks in class (and you can of course contact us to buy one online/by mail) and one or two students in each class will purchase one.
It was sad then to see a First Responder recently passed away, apparently after contracting a disease while performing CPR on a child. It’s hard to know from the information provided just what fatal condition the 3-month-old child could have been carrying. However it is clear from the report that in 2006 the officer performed CPR and ‘came into contact with vomit’ but by the time he had to do CPR again in 2007 he was ‘wearing a protective mask’.
As is often the case in first aid, there’s class-smarts and then there’s experience.
Me? I keep my keychain-mask in my pocket pretty much at all times. I’m also sure I wouldn’t use it on my own children. It’s your call, but make it an informed one.