Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $220.57   13.230%   $249.7514   $249.75   $249.75   $249.80  
  Purchase #2 $162.77   11.156%   $180.9286   $180.93   $180.95   $180.90  
  Purchase #3 $170.98   3.166%   $176.3932   $176.39   $176.40   $176.40  
  Purchase #4 $156.76   8.397%   $169.9231   $169.92   $169.90   $169.90  
  Purchase #5 $99.88   4.491%   $104.3656   $104.37   $104.35   $104.40  
  Purchase #6 $129.72   5.139%   $136.3863   $136.39   $136.40   $136.40  
  Purchase #7 $118.67   11.775%   $132.6434   $132.64   $132.65   $132.60  
  7 purchase totals = $1,059.35       $1,150.3916   $1,150.39   $1,150.40   $1,150.40