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 $17.51   8.439%   $18.9877   $18.99   $19.00   $19.00  
  Purchase #2 $22.71   9.800%   $24.9356   $24.94   $24.95   $24.90  
  Purchase #3 $108.75   4.767%   $113.9341   $113.93   $113.95   $113.90  
  Purchase #4 $131.83   4.321%   $137.5264   $137.53   $137.55   $137.50  
  Purchase #5 $71.29   3.419%   $73.7274   $73.73   $73.75   $73.70  
  Purchase #6 $94.53   9.550%   $103.5576   $103.56   $103.55   $103.60  
  Purchase #7 $140.89   4.365%   $147.0398   $147.04   $147.05   $147.00  
  7 purchase totals = $587.51       $619.7086   $619.72   $619.80   $619.60