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 $18.38   9.566%   $20.1382   $20.14   $20.15   $20.10  
  Purchase #2 $7.68   8.845%   $8.3593   $8.36   $8.35   $8.40  
  Purchase #3 $89.50   10.744%   $99.1159   $99.12   $99.10   $99.10  
  Purchase #4 $222.69   6.173%   $236.4367   $236.44   $236.45   $236.40  
  Purchase #5 $126.69   11.191%   $140.8679   $140.87   $140.85   $140.90  
  Purchase #6 $100.33   7.155%   $107.5086   $107.51   $107.50   $107.50  
  Purchase #7 $15.29   10.725%   $16.9299   $16.93   $16.95   $16.90  
  7 purchase totals = $580.56       $629.3565   $629.37   $629.35   $629.30