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 $122.90   5.523%   $129.6878   $129.69   $129.70   $129.70  
  Purchase #2 $34.34   4.814%   $35.9931   $35.99   $36.00   $36.00  
  Purchase #3 $172.73   4.463%   $180.4389   $180.44   $180.45   $180.40  
  Purchase #4 $78.48   7.577%   $84.4264   $84.43   $84.45   $84.40  
  Purchase #5 $136.79   5.687%   $144.5692   $144.57   $144.55   $144.60  
  Purchase #6 $20.74   13.341%   $23.5069   $23.51   $23.50   $23.50  
  Purchase #7 $44.48   5.761%   $47.0425   $47.04   $47.05   $47.00  
  7 purchase totals = $610.46       $645.6648   $645.67   $645.70   $645.60