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 $12.97   6.600%   $13.826   $13.83   $13.85   $13.80  
  Purchase #2 $19.70   4.436%   $20.5739   $20.57   $20.55   $20.60  
  Purchase #3 $236.55   8.806%   $257.3806   $257.38   $257.40   $257.40  
  Purchase #4 $51.77   9.533%   $56.7052   $56.71   $56.70   $56.70  
  Purchase #5 $11.94   9.496%   $13.0738   $13.07   $13.05   $13.10  
  Purchase #6 $232.62   11.331%   $258.9782   $258.98   $259.00   $259.00  
  Purchase #7 $32.87   4.686%   $34.4103   $34.41   $34.40   $34.40  
  7 purchase totals = $598.42       $654.9480   $654.95   $654.95   $655.00