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 $57.73   3.843%   $59.9486   $59.95   $59.95   $59.90  
  Purchase #2 $148.57   10.744%   $164.5324   $164.53   $164.55   $164.50  
  Purchase #3 $124.59   6.452%   $132.6285   $132.63   $132.65   $132.60  
  Purchase #4 $222.68   10.806%   $246.7428   $246.74   $246.75   $246.70  
  Purchase #5 $238.34   4.213%   $248.3813   $248.38   $248.40   $248.40  
  Purchase #6 $65.25   5.470%   $68.8192   $68.82   $68.80   $68.80  
  Purchase #7 $52.90   12.717%   $59.6273   $59.63   $59.65   $59.60  
  7 purchase totals = $910.06       $980.6801   $980.68   $980.75   $980.50