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 $28.40   5.215%   $29.8811   $29.88   $29.90   $29.90  
  Purchase #2 $71.75   10.449%   $79.2472   $79.25   $79.25   $79.20  
  Purchase #3 $136.44   7.496%   $146.6675   $146.67   $146.65   $146.70  
  Purchase #4 $234.55   12.821%   $264.6217   $264.62   $264.60   $264.60  
  Purchase #5 $7.76   10.302%   $8.5594   $8.56   $8.55   $8.60  
  Purchase #6 $193.57   9.484%   $211.9282   $211.93   $211.95   $211.90  
  Purchase #7 $231.52   7.822%   $249.6295   $249.63   $249.65   $249.60  
  7 purchase totals = $903.99       $990.5346   $990.54   $990.55   $990.50