For various reasons, I just pay mileage at CRA max rate. Simplifies documentation. I do it clean but there are ways to juice the program to increase money transferred to you tax-free from a business. For instance with a multi-vehicle household, one of the vehicles may be used only for work purposes and therefore all mileage would be reimbursed.
If you are a contractor or don't have control of the business reimbursing, that strategy may not work for you. At a past employer, my wife was required to use her vehicle to go to different client sites daily. Work reimbursed mileage (crap rate) and most of her co-workers also wrote off fuel/maintenance/insurance/etc. Yikes. If CRA caught onto one employee, they would burn them all down and it wouldn't be pretty. I kept my wifes filings clean as I don't want to fight CRA.
A former employer chose to provide themselves a monthly car allowance as opposed to paying out specific expenses. That was on the advice of their accountant.
If you are claiming the expenses on your personal income tax, the employer should give you a completed T2200 to legitimize the deduction. T777 is the form you fill out to calculate the deduction.
Use this guide if you are an employee and your employer requires you to pay expenses to earn your employment income.
www.canada.ca