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 $15.94   13.160%   $18.0377   $18.04   $18.05   $18.00  
  Purchase #2 $37.75   3.169%   $38.9463   $38.95   $38.95   $38.90  
  Purchase #3 $163.83   10.874%   $181.6449   $181.64   $181.65   $181.60  
  Purchase #4 $228.28   4.302%   $238.1006   $238.10   $238.10   $238.10  
  Purchase #5 $15.26   5.826%   $16.149   $16.15   $16.15   $16.10  
  Purchase #6 $232.89   8.841%   $253.4798   $253.48   $253.50   $253.50  
  Purchase #7 $226.53   12.861%   $255.664   $255.66   $255.65   $255.70  
  7 purchase totals = $920.48       $1,002.0223   $1,002.02   $1,002.05   $1,001.90