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 $16.80   9.748%   $18.4377   $18.44   $18.45   $18.40  
  Purchase #2 $13.27   13.540%   $15.0668   $15.07   $15.05   $15.10  
  Purchase #3 $141.47   7.323%   $151.8298   $151.83   $151.85   $151.80  
  Purchase #4 $213.79   7.404%   $229.619   $229.62   $229.60   $229.60  
  Purchase #5 $64.77   12.556%   $72.9025   $72.90   $72.90   $72.90  
  Purchase #6 $66.44   4.476%   $69.4139   $69.41   $69.40   $69.40  
  Purchase #7 $12.72   9.547%   $13.9344   $13.93   $13.95   $13.90  
  7 purchase totals = $529.26       $571.2041   $571.20   $571.20   $571.10