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 $246.60   11.602%   $275.2105   $275.21   $275.20   $275.20  
  Purchase #2 $88.59   6.378%   $94.2403   $94.24   $94.25   $94.20  
  Purchase #3 $70.40   5.694%   $74.4086   $74.41   $74.40   $74.40  
  Purchase #4 $40.73   9.724%   $44.6906   $44.69   $44.70   $44.70  
  Purchase #5 $93.32   9.131%   $101.841   $101.84   $101.85   $101.80  
  Purchase #6 $15.27   11.185%   $16.9779   $16.98   $17.00   $17.00  
  Purchase #7 $195.55   13.315%   $221.5875   $221.59   $221.60   $221.60  
  7 purchase totals = $750.46       $828.9564   $828.96   $829.00   $828.90