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 $190.95   6.489%   $203.3407   $203.34   $203.35   $203.30  
  Purchase #2 $18.83   6.170%   $19.9918   $19.99   $20.00   $20.00  
  Purchase #3 $28.56   12.275%   $32.0657   $32.07   $32.05   $32.10  
  Purchase #4 $40.50   4.720%   $42.4116   $42.41   $42.40   $42.40  
  Purchase #5 $51.59   6.780%   $55.0878   $55.09   $55.10   $55.10  
  Purchase #6 $164.85   4.592%   $172.4199   $172.42   $172.40   $172.40  
  Purchase #7 $15.89   10.416%   $17.5451   $17.55   $17.55   $17.50  
  7 purchase totals = $511.17       $542.8626   $542.87   $542.85   $542.80