Or better yet, instead of having extra tax deducted at source set up an automatic RRSP plan with monthly deduction through your bank. Even if you only have $50.00 taken per month, that adds up to $600.00 in year that is earning interest for you, and is a legitimate tax deduction. So come tax time, you also save from $120.00 - $180.00 in taxes (and will get that back if the right amount of tax was taken by your employer).
Many people get confused with allowable deductions because people who are self-employed can claim some expenses they paid in order to earn income. For instance, they can claim meals (at 50%) if they had a dinner meeting with a client. However, the reason for the meeting, who was there, and what was discussed must be well documented. I am a stage actor/teacher and considered self-employed. Most of the contracts I make do not have any tax taken at source. I can claim the business use of my car, business use of part of my home (if using one room as an office), etc. It may sound great, but it really costs more than being an employee. There is no coverage for sick days or holidays (if you don't work you don't earn anything), a self-employed person has to pay both the employee AND employer portion of CPP - that's double, and even though we don't have to pay EI premiums, we will never get EI benefits. So if your business is slow or goes belly up... oh well.
It is much better to err on the side of caution when it comes to deductions at tax time. Or at least get advice from a professional. Many people have claimed things and have not been caught....yet. If you are audited (they can go back six years plus the current year) and if they find errors or ineligible deductions, your taxes will be adjusted and you will repay that amount including interest for all those years and a penalty on top of that. And THAT'S assuming that they don't think you purposely committed fraud.