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 $116.69   12.424%   $131.1876   $131.19   $131.20   $131.20  
  Purchase #2 $236.77   7.673%   $254.9374   $254.94   $254.95   $254.90  
  Purchase #3 $59.41   8.428%   $64.4171   $64.42   $64.40   $64.40  
  Purchase #4 $191.84   3.785%   $199.1011   $199.10   $199.10   $199.10  
  Purchase #5 $112.69   10.660%   $124.7028   $124.70   $124.70   $124.70  
  Purchase #6 $58.62   4.430%   $61.2169   $61.22   $61.20   $61.20  
  Purchase #7 $119.92   13.520%   $136.1332   $136.13   $136.15   $136.10  
  7 purchase totals = $895.94       $971.6961   $971.70   $971.70   $971.60