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 $10.29   5.128%   $10.8177   $10.82   $10.80   $10.80  
  Purchase #2 $28.66   9.247%   $31.3102   $31.31   $31.30   $31.30  
  Purchase #3 $109.76   5.127%   $115.3874   $115.39   $115.40   $115.40  
  Purchase #4 $107.67   9.126%   $117.496   $117.50   $117.50   $117.50  
  Purchase #5 $19.69   6.174%   $20.9057   $20.91   $20.90   $20.90  
  Purchase #6 $168.30   3.827%   $174.7408   $174.74   $174.75   $174.70  
  Purchase #7 $154.82   9.468%   $169.4784   $169.48   $169.50   $169.50  
  7 purchase totals = $599.19       $640.1362   $640.15   $640.15   $640.10