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 $17.40   3.665%   $18.0377   $18.04   $18.05   $18.00  
  Purchase #2 $145.49   10.305%   $160.4827   $160.48   $160.50   $160.50  
  Purchase #3 $209.71   12.319%   $235.5442   $235.54   $235.55   $235.50  
  Purchase #4 $86.45   4.105%   $89.9988   $90.00   $90.00   $90.00  
  Purchase #5 $244.87   5.347%   $257.9632   $257.96   $257.95   $258.00  
  Purchase #6 $215.28   10.440%   $237.7552   $237.76   $237.75   $237.80  
  Purchase #7 $34.62   7.682%   $37.2795   $37.28   $37.30   $37.30  
  7 purchase totals = $953.82       $1,037.0613   $1,037.06   $1,037.10   $1,037.10