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 $51.73   13.396%   $58.6598   $58.66   $58.65   $58.70  
  Purchase #2 $228.68   4.597%   $239.1924   $239.19   $239.20   $239.20  
  Purchase #3 $87.88   10.358%   $96.9826   $96.98   $97.00   $97.00  
  Purchase #4 $151.40   5.854%   $160.263   $160.26   $160.25   $160.30  
  Purchase #5 $195.31   12.230%   $219.1964   $219.20   $219.20   $219.20  
  Purchase #6 $31.29   3.855%   $32.4962   $32.50   $32.50   $32.50  
  Purchase #7 $76.98   7.650%   $82.869   $82.87   $82.85   $82.90  
  7 purchase totals = $823.27       $889.6594   $889.66   $889.65   $889.80