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 $94.47   6.334%   $100.4537   $100.45   $100.45   $100.50  
  Purchase #2 $41.69   13.694%   $47.399   $47.40   $47.40   $47.40  
  Purchase #3 $90.83   7.420%   $97.5696   $97.57   $97.55   $97.60  
  Purchase #4 $11.30   3.767%   $11.7257   $11.73   $11.75   $11.70  
  Purchase #5 $118.39   12.621%   $133.332   $133.33   $133.35   $133.30  
  Purchase #6 $105.29   8.479%   $114.2175   $114.22   $114.20   $114.20  
  Purchase #7 $149.75   3.813%   $155.46   $155.46   $155.45   $155.50  
  7 purchase totals = $611.72       $660.1575   $660.16   $660.15   $660.20