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 $19.64   6.677%   $20.9514   $20.95   $20.95   $21.00  
  Purchase #2 $148.97   6.560%   $158.7424   $158.74   $158.75   $158.70  
  Purchase #3 $122.41   3.795%   $127.0555   $127.06   $127.05   $127.10  
  Purchase #4 $38.62   8.688%   $41.9753   $41.98   $42.00   $42.00  
  Purchase #5 $138.77   13.267%   $157.1806   $157.18   $157.20   $157.20  
  Purchase #6 $140.56   10.698%   $155.5971   $155.60   $155.60   $155.60  
  Purchase #7 $140.48   10.455%   $155.1672   $155.17   $155.15   $155.20  
  7 purchase totals = $749.45       $816.6695   $816.68   $816.70   $816.80