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 $107.47   13.828%   $122.331   $122.33   $122.35   $122.30  
  Purchase #2 $52.38   6.798%   $55.9408   $55.94   $55.95   $55.90  
  Purchase #3 $116.90   3.656%   $121.1739   $121.17   $121.15   $121.20  
  Purchase #4 $12.74   4.217%   $13.2772   $13.28   $13.30   $13.30  
  Purchase #5 $24.61   12.261%   $27.6274   $27.63   $27.65   $27.60  
  Purchase #6 $140.96   5.519%   $148.7396   $148.74   $148.75   $148.70  
  Purchase #7 $132.87   13.678%   $151.044   $151.04   $151.05   $151.00  
  7 purchase totals = $587.93       $640.1339   $640.13   $640.20   $640.00