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 $227.91   5.827%   $241.1903   $241.19   $241.20   $241.20  
  Purchase #2 $37.68   13.642%   $42.8203   $42.82   $42.80   $42.80  
  Purchase #3 $24.55   8.423%   $26.6178   $26.62   $26.60   $26.60  
  Purchase #4 $208.29   6.874%   $222.6079   $222.61   $222.60   $222.60  
  Purchase #5 $43.86   13.124%   $49.6162   $49.62   $49.60   $49.60  
  Purchase #6 $50.44   13.792%   $57.3967   $57.40   $57.40   $57.40  
  Purchase #7 $207.65   3.410%   $214.7309   $214.73   $214.75   $214.70  
  7 purchase totals = $800.38       $854.9801   $854.99   $854.95   $854.90