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 $19.44   9.514%   $21.2895   $21.29   $21.30   $21.30  
  Purchase #2 $187.35   6.217%   $198.9975   $199.00   $199.00   $199.00  
  Purchase #3 $45.76   12.515%   $51.4869   $51.49   $51.50   $51.50  
  Purchase #4 $5.47   13.838%   $6.2269   $6.23   $6.25   $6.20  
  Purchase #5 $93.81   12.748%   $105.7689   $105.77   $105.75   $105.80  
  Purchase #6 $17.88   10.141%   $19.6932   $19.69   $19.70   $19.70  
  Purchase #7 $224.56   6.489%   $239.1317   $239.13   $239.15   $239.10  
  7 purchase totals = $594.27       $642.5946   $642.60   $642.65   $642.60