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 $58.85   12.544%   $66.2321   $66.23   $66.25   $66.20  
  Purchase #2 $231.93   5.815%   $245.4167   $245.42   $245.40   $245.40  
  Purchase #3 $147.40   5.584%   $155.6308   $155.63   $155.65   $155.60  
  Purchase #4 $43.67   7.692%   $47.0291   $47.03   $47.05   $47.00  
  Purchase #5 $203.97   4.660%   $213.475   $213.48   $213.50   $213.50  
  Purchase #6 $35.76   12.246%   $40.1392   $40.14   $40.15   $40.10  
  Purchase #7 $191.65   11.281%   $213.27   $213.27   $213.25   $213.30  
  7 purchase totals = $913.23       $981.1929   $981.20   $981.25   $981.10