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 $18.90   9.621%   $20.7184   $20.72   $20.70   $20.70  
  Purchase #2 $237.81   13.583%   $270.1117   $270.11   $270.10   $270.10  
  Purchase #3 $77.82   7.128%   $83.367   $83.37   $83.35   $83.40  
  Purchase #4 $7.45   5.664%   $7.872   $7.87   $7.85   $7.90  
  Purchase #5 $213.68   6.654%   $227.8983   $227.90   $227.90   $227.90  
  Purchase #6 $188.97   12.780%   $213.1204   $213.12   $213.10   $213.10  
  Purchase #7 $62.78   9.506%   $68.7479   $68.75   $68.75   $68.70  
  7 purchase totals = $807.41       $891.8357   $891.84   $891.75   $891.80