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 $27.57   11.783%   $30.8186   $30.82   $30.80   $30.80  
  Purchase #2 $59.99   4.581%   $62.7381   $62.74   $62.75   $62.70  
  Purchase #3 $9.27   7.104%   $9.9285   $9.93   $9.95   $9.90  
  Purchase #4 $234.96   9.867%   $258.1435   $258.14   $258.15   $258.10  
  Purchase #5 $178.57   11.364%   $198.8627   $198.86   $198.85   $198.90  
  Purchase #6 $98.27   11.192%   $109.2684   $109.27   $109.25   $109.30  
  Purchase #7 $185.71   8.489%   $201.4749   $201.47   $201.45   $201.50  
  7 purchase totals = $794.34       $871.2347   $871.23   $871.20   $871.20