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 $66.95   10.402%   $73.9141   $73.91   $73.90   $73.90  
  Purchase #2 $179.81   12.540%   $202.3582   $202.36   $202.35   $202.40  
  Purchase #3 $91.35   8.341%   $98.9695   $98.97   $98.95   $99.00  
  Purchase #4 $59.63   4.760%   $62.4684   $62.47   $62.45   $62.50  
  Purchase #5 $243.29   12.425%   $273.5188   $273.52   $273.50   $273.50  
  Purchase #6 $116.68   11.315%   $129.8823   $129.88   $129.90   $129.90  
  Purchase #7 $139.56   13.560%   $158.4843   $158.48   $158.50   $158.50  
  7 purchase totals = $897.27       $999.5956   $999.59   $999.55   $999.70