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 $236.56   9.104%   $258.0964   $258.10   $258.10   $258.10  
  Purchase #2 $119.70   6.217%   $127.1417   $127.14   $127.15   $127.10  
  Purchase #3 $178.39   13.125%   $201.8037   $201.80   $201.80   $201.80  
  Purchase #4 $29.69   4.107%   $30.9094   $30.91   $30.90   $30.90  
  Purchase #5 $51.77   10.474%   $57.1924   $57.19   $57.20   $57.20  
  Purchase #6 $154.59   5.610%   $163.2625   $163.26   $163.25   $163.30  
  Purchase #7 $159.49   3.139%   $164.4964   $164.50   $164.50   $164.50  
  7 purchase totals = $930.19       $1,002.9025   $1,002.90   $1,002.90   $1,002.90