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 $28.80   6.655%   $30.7166   $30.72   $30.70   $30.70  
  Purchase #2 $94.60   4.580%   $98.9327   $98.93   $98.95   $98.90  
  Purchase #3 $238.44   10.418%   $263.2807   $263.28   $263.30   $263.30  
  Purchase #4 $126.67   8.223%   $137.0861   $137.09   $137.10   $137.10  
  Purchase #5 $124.97   7.272%   $134.0578   $134.06   $134.05   $134.10  
  Purchase #6 $83.58   6.684%   $89.1665   $89.17   $89.15   $89.20  
  Purchase #7 $139.85   13.821%   $159.1787   $159.18   $159.20   $159.20  
  7 purchase totals = $836.91       $912.4191   $912.43   $912.45   $912.50