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 $17.75   3.483%   $18.3682   $18.37   $18.35   $18.40  
  Purchase #2 $9.89   7.494%   $10.6312   $10.63   $10.65   $10.60  
  Purchase #3 $23.66   3.474%   $24.4819   $24.48   $24.50   $24.50  
  Purchase #4 $196.47   6.771%   $209.773   $209.77   $209.75   $209.80  
  Purchase #5 $142.34   13.753%   $161.916   $161.92   $161.90   $161.90  
  Purchase #6 $35.37   7.783%   $38.1228   $38.12   $38.10   $38.10  
  Purchase #7 $129.83   9.680%   $142.3975   $142.40   $142.40   $142.40  
  7 purchase totals = $555.31       $605.6906   $605.69   $605.65   $605.70