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 $19.99   7.384%   $21.4661   $21.47   $21.45   $21.50  
  Purchase #2 $68.68   12.293%   $77.1228   $77.12   $77.10   $77.10  
  Purchase #3 $97.55   8.503%   $105.8447   $105.84   $105.85   $105.80  
  Purchase #4 $206.95   4.484%   $216.2296   $216.23   $216.25   $216.20  
  Purchase #5 $146.70   12.342%   $164.8057   $164.81   $164.80   $164.80  
  Purchase #6 $230.26   3.355%   $237.9852   $237.99   $238.00   $238.00  
  Purchase #7 $24.57   13.469%   $27.8793   $27.88   $27.90   $27.90  
  7 purchase totals = $794.70       $851.3334   $851.34   $851.35   $851.30