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 $25.91   7.400%   $27.8273   $27.83   $27.85   $27.80  
  Purchase #2 $69.40   13.811%   $78.9848   $78.98   $79.00   $79.00  
  Purchase #3 $216.39   7.139%   $231.8381   $231.84   $231.85   $231.80  
  Purchase #4 $114.88   6.583%   $122.4426   $122.44   $122.45   $122.40  
  Purchase #5 $185.44   12.603%   $208.811   $208.81   $208.80   $208.80  
  Purchase #6 $110.48   8.712%   $120.105   $120.11   $120.10   $120.10  
  Purchase #7 $6.31   3.453%   $6.5279   $6.53   $6.55   $6.50  
  7 purchase totals = $728.81       $796.5367   $796.54   $796.60   $796.40