Mileage Reimbursment - what is covered 'on the way to work'?

Some of our staff use their morning/afternoon commute to stop by clients offices and and review items or pick things up. They claim they are making a business trip to do this and believe they should be reimbursed mileage for the entire trip even if this only adds 2-3 miles to their regular commute. At a previous company I worked for we allowed for mileage reimbursement based upon the difference between their regular commute (no reimbursement, feel free to put on your tax returns), and the extra miles it took to get to the second location. Backup in the form of Mapquest directions showing the distances was required with their expense reports.

I am trying to determine what common practice is and if there is somewhere I could locate solid backup to support a company policy here for this. We don't want to discourage this practice, as it is the best use of time when appropriate, but it seems unfair to be reimbursed for your regular morning drive simply by stopping  somewhere to say hello, and encourages abuse.