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 $225.68   12.318%   $253.4793   $253.48   $253.50   $253.50  
  Purchase #2 $53.29   10.171%   $58.7101   $58.71   $58.70   $58.70  
  Purchase #3 $26.79   3.452%   $27.7148   $27.71   $27.70   $27.70  
  Purchase #4 $10.80   4.235%   $11.2574   $11.26   $11.25   $11.30  
  Purchase #5 $237.93   12.461%   $267.5785   $267.58   $267.60   $267.60  
  Purchase #6 $66.25   9.127%   $72.2966   $72.30   $72.30   $72.30  
  Purchase #7 $64.52   6.781%   $68.8951   $68.90   $68.90   $68.90  
  7 purchase totals = $685.26       $759.9318   $759.94   $759.95   $760.00