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 $45.51   12.287%   $51.1018   $51.10   $51.10   $51.10  
  Purchase #2 $99.98   3.383%   $103.3623   $103.36   $103.35   $103.40  
  Purchase #3 $27.35   6.435%   $29.11   $29.11   $29.10   $29.10  
  Purchase #4 $91.52   6.556%   $97.5201   $97.52   $97.50   $97.50  
  Purchase #5 $91.40   5.276%   $96.2223   $96.22   $96.20   $96.20  
  Purchase #6 $199.62   3.676%   $206.958   $206.96   $206.95   $207.00  
  Purchase #7 $73.65   6.209%   $78.2229   $78.22   $78.20   $78.20  
  7 purchase totals = $629.03       $662.4974   $662.49   $662.40   $662.50