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 $145.71   13.421%   $165.2657   $165.27   $165.25   $165.30  
  Purchase #2 $234.76   9.841%   $257.8627   $257.86   $257.85   $257.90  
  Purchase #3 $46.35   8.185%   $50.1437   $50.14   $50.15   $50.10  
  Purchase #4 $87.73   7.863%   $94.6282   $94.63   $94.65   $94.60  
  Purchase #5 $219.36   13.503%   $248.9802   $248.98   $249.00   $249.00  
  Purchase #6 $153.98   9.787%   $169.05   $169.05   $169.05   $169.10  
  Purchase #7 $27.48   6.655%   $29.3088   $29.31   $29.30   $29.30  
  7 purchase totals = $915.37       $1,015.2393   $1,015.24   $1,015.25   $1,015.30