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 $17.73   3.499%   $18.3504   $18.35   $18.35   $18.40  
  Purchase #2 $105.58   7.770%   $113.7836   $113.78   $113.80   $113.80  
  Purchase #3 $218.42   5.700%   $230.8699   $230.87   $230.85   $230.90  
  Purchase #4 $117.43   9.752%   $128.8818   $128.88   $128.90   $128.90  
  Purchase #5 $134.66   6.701%   $143.6836   $143.68   $143.70   $143.70  
  Purchase #6 $39.72   11.619%   $44.3351   $44.34   $44.35   $44.30  
  Purchase #7 $114.81   13.728%   $130.5711   $130.57   $130.55   $130.60  
  7 purchase totals = $748.35       $810.4755   $810.47   $810.50   $810.60