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 $229.47   12.257%   $257.5961   $257.60   $257.60   $257.60  
  Purchase #2 $187.82   3.171%   $193.7758   $193.78   $193.80   $193.80  
  Purchase #3 $47.26   13.105%   $53.4534   $53.45   $53.45   $53.50  
  Purchase #4 $151.48   7.472%   $162.7986   $162.80   $162.80   $162.80  
  Purchase #5 $124.95   13.841%   $142.2443   $142.24   $142.25   $142.20  
  Purchase #6 $50.55   3.175%   $52.155   $52.16   $52.15   $52.20  
  Purchase #7 $141.61   12.496%   $159.3056   $159.31   $159.30   $159.30  
  7 purchase totals = $933.14       $1,021.3288   $1,021.34   $1,021.35   $1,021.40