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 $19.58   4.156%   $20.3937   $20.39   $20.40   $20.40  
  Purchase #2 $24.37   7.485%   $26.1941   $26.19   $26.20   $26.20  
  Purchase #3 $63.74   12.874%   $71.9459   $71.95   $71.95   $71.90  
  Purchase #4 $194.98   10.388%   $215.2345   $215.23   $215.25   $215.20  
  Purchase #5 $78.70   3.750%   $81.6513   $81.65   $81.65   $81.70  
  Purchase #6 $86.64   7.499%   $93.1371   $93.14   $93.15   $93.10  
  Purchase #7 $129.68   5.641%   $136.9952   $137.00   $137.00   $137.00  
  7 purchase totals = $597.69       $645.5518   $645.55   $645.60   $645.50