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 $239.27   8.544%   $259.7132   $259.71   $259.70   $259.70  
  Purchase #2 $166.83   5.610%   $176.1892   $176.19   $176.20   $176.20  
  Purchase #3 $15.29   4.293%   $15.9464   $15.95   $15.95   $15.90  
  Purchase #4 $244.91   10.354%   $270.268   $270.27   $270.25   $270.30  
  Purchase #5 $192.90   6.675%   $205.7761   $205.78   $205.80   $205.80  
  Purchase #6 $34.86   11.851%   $38.9913   $38.99   $39.00   $39.00  
  Purchase #7 $63.68   12.280%   $71.4999   $71.50   $71.50   $71.50  
  7 purchase totals = $957.74       $1,038.3841   $1,038.39   $1,038.40   $1,038.40