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 $238.55   12.735%   $268.9293   $268.93   $268.95   $268.90  
  Purchase #2 $210.31   3.663%   $218.0137   $218.01   $218.00   $218.00  
  Purchase #3 $43.65   11.486%   $48.6636   $48.66   $48.65   $48.70  
  Purchase #4 $59.39   10.665%   $65.7239   $65.72   $65.70   $65.70  
  Purchase #5 $177.36   11.268%   $197.3449   $197.34   $197.35   $197.30  
  Purchase #6 $40.96   11.469%   $45.6577   $45.66   $45.65   $45.70  
  Purchase #7 $117.72   10.158%   $129.678   $129.68   $129.70   $129.70  
  7 purchase totals = $887.94       $974.0111   $974.00   $974.00   $974.00