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 $59.37   12.137%   $66.5757   $66.58   $66.60   $66.60  
  Purchase #2 $44.55   4.160%   $46.4033   $46.40   $46.40   $46.40  
  Purchase #3 $62.64   12.211%   $70.289   $70.29   $70.30   $70.30  
  Purchase #4 $107.26   11.708%   $119.818   $119.82   $119.80   $119.80  
  Purchase #5 $24.81   9.527%   $27.1736   $27.17   $27.15   $27.20  
  Purchase #6 $228.75   4.494%   $239.03   $239.03   $239.05   $239.00  
  Purchase #7 $12.92   11.300%   $14.38   $14.38   $14.40   $14.40  
  7 purchase totals = $540.30       $583.6696   $583.67   $583.70   $583.70