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 $19.87   7.238%   $21.3082   $21.31   $21.30   $21.30  
  Purchase #2 $62.25   13.560%   $70.6911   $70.69   $70.70   $70.70  
  Purchase #3 $237.32   12.346%   $266.6195   $266.62   $266.60   $266.60  
  Purchase #4 $49.75   3.379%   $51.4311   $51.43   $51.45   $51.40  
  Purchase #5 $235.48   5.817%   $249.1779   $249.18   $249.20   $249.20  
  Purchase #6 $133.36   5.235%   $140.3414   $140.34   $140.35   $140.30  
  Purchase #7 $43.65   10.270%   $48.1329   $48.13   $48.15   $48.10  
  7 purchase totals = $781.68       $847.7021   $847.70   $847.75   $847.60