My understanding of the law (and we have a labor lawyer on retainer) is that if your employee handbook specifically states that you can take money from the employee in the case of negligence, and if the employee signs that he got the handbook and agrees to abide by your policies - then it is legal to deduct the money as long as you don't take the amount left for the employee down under minimum wage (Federal law is specific that all employees MUST get minimum wage no matter what).
However, as noted above - if you start deducting it and the employee quits.... you're SOL.
Interesting separate issue (somewhat along these lines).... we have a clause in our handbook that if an hourly employee quits without giving two weeks notice, then any wages due him/her are then paid at "minimum wage". We have had this for a few years now. We pay every two weeks, so this can mean that as much as two weeks pay is done at 'minimum wage'. We had it challenged recently (by two employees that quit without notice, got their last paycheck and went to the labor board bitchin up a storm). Labor Board went to bat for them and instructed us to pay them the actual wage they were at when they quit. We fought it (with our labor lawyer) and won on appeal. It was upheld as legal because the employees agreed to in by signing for the handbook and agreeing to the contents contained therein, and we didn't take them below minimum wage as required by federal law. Grumbling stopped when THAT got out amongst the troops.