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 $18.99   11.740%   $21.2194   $21.22   $21.20   $21.20  
  Purchase #2 $232.57   4.356%   $242.7007   $242.70   $242.70   $242.70  
  Purchase #3 $50.65   8.152%   $54.779   $54.78   $54.80   $54.80  
  Purchase #4 $226.83   5.166%   $238.548   $238.55   $238.55   $238.50  
  Purchase #5 $162.33   10.721%   $179.7334   $179.73   $179.75   $179.70  
  Purchase #6 $83.75   10.609%   $92.635   $92.64   $92.65   $92.60  
  Purchase #7 $67.55   12.437%   $75.9512   $75.95   $75.95   $76.00  
  7 purchase totals = $842.67       $905.5667   $905.57   $905.60   $905.50