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 $18.30   13.719%   $20.8106   $20.81   $20.80   $20.80  
  Purchase #2 $99.40   9.802%   $109.1432   $109.14   $109.15   $109.10  
  Purchase #3 $118.44   11.182%   $131.684   $131.68   $131.70   $131.70  
  Purchase #4 $6.58   9.536%   $7.2075   $7.21   $7.20   $7.20  
  Purchase #5 $247.29   9.546%   $270.8963   $270.90   $270.90   $270.90  
  Purchase #6 $197.28   10.138%   $217.2802   $217.28   $217.30   $217.30  
  Purchase #7 $138.47   8.455%   $150.1776   $150.18   $150.20   $150.20  
  7 purchase totals = $825.76       $907.1994   $907.20   $907.25   $907.20