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 $159.56   12.492%   $179.4922   $179.49   $179.50   $179.50  
  Purchase #2 $10.83   4.861%   $11.3564   $11.36   $11.35   $11.40  
  Purchase #3 $118.96   12.744%   $134.1203   $134.12   $134.10   $134.10  
  Purchase #4 $136.73   9.584%   $149.8342   $149.83   $149.85   $149.80  
  Purchase #5 $217.62   13.798%   $247.6472   $247.65   $247.65   $247.60  
  Purchase #6 $47.38   9.418%   $51.8422   $51.84   $51.85   $51.80  
  Purchase #7 $40.30   3.302%   $41.6307   $41.63   $41.65   $41.60  
  7 purchase totals = $731.38       $815.9232   $815.92   $815.95   $815.80