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 $17.96   4.413%   $18.7526   $18.75   $18.75   $18.80  
  Purchase #2 $25.46   12.840%   $28.7291   $28.73   $28.75   $28.70  
  Purchase #3 $13.40   13.409%   $15.1968   $15.20   $15.20   $15.20  
  Purchase #4 $109.54   12.347%   $123.0649   $123.06   $123.05   $123.10  
  Purchase #5 $24.32   13.294%   $27.5531   $27.55   $27.55   $27.60  
  Purchase #6 $250.95   13.411%   $284.6049   $284.60   $284.60   $284.60  
  Purchase #7 $81.34   3.538%   $84.2178   $84.22   $84.20   $84.20  
  7 purchase totals = $522.97       $582.1192   $582.11   $582.10   $582.20