Hmm... I suppose technically, rent is income and therefore, you technically may have to report it... at a quick look, and it seems that it depends a bit on your set-up.... if most of your house is being rented, then you might need to.
T4036 - Rental Income - 2008
How would they know? Who knows? Maybe you'll get audited. Maybe your roommate will claim rent on their taxes. I don't think it's a likely case that they'll dig into it, but in any case, Revenue Canada does not toss you in jail for this, they take the money.
Now, the advantage of reporting it is that you could write off some of your major household repairs or expenses.
Take a look here under Personal Portion:
T4036 - Rental Income - 2008
Interestingly, your roommates would not be governed under the Residential Tenancies Act (in Ontario--and probably not under equivalent law in other provinces) if they are sharing a kitchen and/or bathroom with you.
As for insurance, I think you are best off insuring everything--house/contents insurance isn't that expensive. Since you aren't governed by the Act, you should be able to cover that with a written agreement. But it might not hurt to talk to your insurance agent to see if your roommates could just get tenant's insurance on their own rooms, and then let them worry about it.
Seem that each group (CRA, law, insurers) define a rental property or a tenant a little differently.