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 $18.46   13.677%   $20.9848   $20.98   $21.00   $21.00  
  Purchase #2 $166.95   11.675%   $186.4414   $186.44   $186.45   $186.40  
  Purchase #3 $19.31   9.128%   $21.0726   $21.07   $21.05   $21.10  
  Purchase #4 $187.75   10.698%   $207.8355   $207.84   $207.85   $207.80  
  Purchase #5 $112.93   13.603%   $128.2919   $128.29   $128.30   $128.30  
  Purchase #6 $183.28   7.543%   $197.1048   $197.10   $197.10   $197.10  
  Purchase #7 $174.67   9.185%   $190.7134   $190.71   $190.70   $190.70  
  7 purchase totals = $863.35       $952.4444   $952.43   $952.45   $952.40