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 $246.86   4.227%   $257.2948   $257.29   $257.30   $257.30  
  Purchase #2 $151.41   9.579%   $165.9136   $165.91   $165.90   $165.90  
  Purchase #3 $69.54   10.580%   $76.8973   $76.90   $76.90   $76.90  
  Purchase #4 $165.60   7.301%   $177.6905   $177.69   $177.70   $177.70  
  Purchase #5 $244.91   5.809%   $259.1368   $259.14   $259.15   $259.10  
  Purchase #6 $7.64   12.800%   $8.6179   $8.62   $8.60   $8.60  
  Purchase #7 $60.31   12.417%   $67.7987   $67.80   $67.80   $67.80  
  7 purchase totals = $946.27       $1,013.3496   $1,013.35   $1,013.35   $1,013.30