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 $29.90   3.536%   $30.9573   $30.96   $30.95   $31.00  
  Purchase #2 $30.60   6.447%   $32.5728   $32.57   $32.55   $32.60  
  Purchase #3 $112.68   7.740%   $121.4014   $121.40   $121.40   $121.40  
  Purchase #4 $102.33   12.826%   $115.4548   $115.45   $115.45   $115.50  
  Purchase #5 $102.71   12.671%   $115.7244   $115.72   $115.70   $115.70  
  Purchase #6 $149.70   8.500%   $162.4245   $162.42   $162.40   $162.40  
  Purchase #7 $188.30   4.840%   $197.4137   $197.41   $197.40   $197.40  
  7 purchase totals = $716.22       $775.9489   $775.93   $775.85   $776.00