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 $171.88   12.130%   $192.729   $192.73   $192.75   $192.70  
  Purchase #2 $106.63   11.810%   $119.223   $119.22   $119.20   $119.20  
  Purchase #3 $7.92   3.306%   $8.1818   $8.18   $8.20   $8.20  
  Purchase #4 $134.69   4.406%   $140.6244   $140.62   $140.60   $140.60  
  Purchase #5 $58.64   13.459%   $66.5324   $66.53   $66.55   $66.50  
  Purchase #6 $20.75   3.849%   $21.5487   $21.55   $21.55   $21.50  
  Purchase #7 $87.81   4.277%   $91.5656   $91.57   $91.55   $91.60  
  7 purchase totals = $588.32       $640.4049   $640.40   $640.40   $640.30