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 $241.58   13.314%   $273.744   $273.74   $273.75   $273.70  
  Purchase #2 $115.94   5.856%   $122.7294   $122.73   $122.75   $122.70  
  Purchase #3 $179.63   12.739%   $202.5131   $202.51   $202.50   $202.50  
  Purchase #4 $154.97   8.777%   $168.5717   $168.57   $168.55   $168.60  
  Purchase #5 $208.83   5.776%   $220.892   $220.89   $220.90   $220.90  
  Purchase #6 $49.69   5.523%   $52.4344   $52.43   $52.45   $52.40  
  Purchase #7 $213.30   11.657%   $238.1644   $238.16   $238.15   $238.20  
  7 purchase totals = $1,163.94       $1,279.0490   $1,279.03   $1,279.05   $1,279.00