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 $139.79   6.854%   $149.3712   $149.37   $149.35   $149.40  
  Purchase #2 $122.91   6.703%   $131.1487   $131.15   $131.15   $131.10  
  Purchase #3 $138.89   5.408%   $146.4012   $146.40   $146.40   $146.40  
  Purchase #4 $6.40   10.293%   $7.0588   $7.06   $7.05   $7.10  
  Purchase #5 $142.76   6.674%   $152.2878   $152.29   $152.30   $152.30  
  Purchase #6 $194.73   10.805%   $215.7706   $215.77   $215.75   $215.80  
  Purchase #7 $152.37   11.186%   $169.4141   $169.41   $169.40   $169.40  
  7 purchase totals = $897.85       $971.4524   $971.45   $971.40   $971.50