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 $25.83   8.719%   $28.0821   $28.08   $28.10   $28.10  
  Purchase #2 $9.73   3.695%   $10.0895   $10.09   $10.10   $10.10  
  Purchase #3 $100.27   12.308%   $112.6112   $112.61   $112.60   $112.60  
  Purchase #4 $199.97   6.131%   $212.2302   $212.23   $212.25   $212.20  
  Purchase #5 $17.27   5.451%   $18.2114   $18.21   $18.20   $18.20  
  Purchase #6 $93.55   12.647%   $105.3813   $105.38   $105.40   $105.40  
  Purchase #7 $131.78   12.431%   $148.1616   $148.16   $148.15   $148.20  
  7 purchase totals = $578.40       $634.7673   $634.76   $634.80   $634.80