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 $36.96   7.336%   $39.6714   $39.67   $39.65   $39.70  
  Purchase #2 $102.63   5.330%   $108.1002   $108.10   $108.10   $108.10  
  Purchase #3 $11.62   9.110%   $12.6786   $12.68   $12.70   $12.70  
  Purchase #4 $73.59   11.273%   $81.8858   $81.89   $81.90   $81.90  
  Purchase #5 $92.57   13.574%   $105.1355   $105.14   $105.15   $105.10  
  Purchase #6 $126.90   7.588%   $136.5292   $136.53   $136.55   $136.50  
  Purchase #7 $136.57   4.822%   $143.1554   $143.16   $143.15   $143.20  
  7 purchase totals = $580.84       $627.1561   $627.17   $627.20   $627.20