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 $17.36   4.110%   $18.0735   $18.07   $18.05   $18.10  
  Purchase #2 $235.50   5.316%   $248.0192   $248.02   $248.00   $248.00  
  Purchase #3 $244.55   11.748%   $273.2797   $273.28   $273.30   $273.30  
  Purchase #4 $139.87   11.174%   $155.4991   $155.50   $155.50   $155.50  
  Purchase #5 $25.45   9.368%   $27.8342   $27.83   $27.85   $27.80  
  Purchase #6 $17.57   6.225%   $18.6637   $18.66   $18.65   $18.70  
  Purchase #7 $225.27   5.675%   $238.0541   $238.05   $238.05   $238.10  
  7 purchase totals = $905.57       $979.4235   $979.41   $979.40   $979.50