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 $17.42   3.742%   $18.0719   $18.07   $18.05   $18.10  
  Purchase #2 $124.56   7.839%   $134.3243   $134.32   $134.30   $134.30  
  Purchase #3 $87.44   13.277%   $99.0494   $99.05   $99.05   $99.00  
  Purchase #4 $207.29   11.377%   $230.8734   $230.87   $230.85   $230.90  
  Purchase #5 $77.47   3.808%   $80.4201   $80.42   $80.40   $80.40  
  Purchase #6 $124.27   5.405%   $130.9868   $130.99   $131.00   $131.00  
  Purchase #7 $129.74   12.560%   $146.0353   $146.04   $146.05   $146.00  
  7 purchase totals = $768.19       $839.7612   $839.76   $839.70   $839.70