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 $21.44   6.625%   $22.8604   $22.86   $22.85   $22.90  
  Purchase #2 $64.55   6.545%   $68.7748   $68.77   $68.75   $68.80  
  Purchase #3 $125.44   8.604%   $136.2329   $136.23   $136.25   $136.20  
  Purchase #4 $14.38   12.325%   $16.1523   $16.15   $16.15   $16.20  
  Purchase #5 $189.90   10.367%   $209.5869   $209.59   $209.60   $209.60  
  Purchase #6 $226.86   7.663%   $244.2443   $244.24   $244.25   $244.20  
  Purchase #7 $129.71   9.593%   $142.1531   $142.15   $142.15   $142.20  
  7 purchase totals = $772.28       $840.0047   $839.99   $840.00   $840.10