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 $229.80   8.130%   $248.4827   $248.48   $248.50   $248.50  
  Purchase #2 $214.84   6.659%   $229.1462   $229.15   $229.15   $229.10  
  Purchase #3 $50.94   9.189%   $55.6209   $55.62   $55.60   $55.60  
  Purchase #4 $58.70   7.165%   $62.9059   $62.91   $62.90   $62.90  
  Purchase #5 $233.87   6.452%   $248.9593   $248.96   $248.95   $249.00  
  Purchase #6 $46.69   8.401%   $50.6124   $50.61   $50.60   $50.60  
  Purchase #7 $8.68   6.585%   $9.2516   $9.25   $9.25   $9.30  
  7 purchase totals = $843.52       $904.9790   $904.98   $904.95   $905.00