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 $7.74   8.270%   $8.3801   $8.38   $8.40   $8.40  
  Purchase #2 $249.36   4.573%   $260.7632   $260.76   $260.75   $260.80  
  Purchase #3 $218.75   4.860%   $229.3813   $229.38   $229.40   $229.40  
  Purchase #4 $36.78   4.559%   $38.4568   $38.46   $38.45   $38.50  
  Purchase #5 $17.58   6.735%   $18.764   $18.76   $18.75   $18.80  
  Purchase #6 $7.45   5.722%   $7.8763   $7.88   $7.90   $7.90  
  Purchase #7 $100.56   3.801%   $104.3823   $104.38   $104.40   $104.40  
  7 purchase totals = $638.22       $668.0040   $668.00   $668.05   $668.20