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 $242.73   13.724%   $276.0423   $276.04   $276.05   $276.00  
  Purchase #2 $94.69   6.406%   $100.7558   $100.76   $100.75   $100.80  
  Purchase #3 $123.41   5.453%   $130.1395   $130.14   $130.15   $130.10  
  Purchase #4 $71.25   5.840%   $75.411   $75.41   $75.40   $75.40  
  Purchase #5 $104.82   3.567%   $108.5589   $108.56   $108.55   $108.60  
  Purchase #6 $235.89   12.839%   $266.1759   $266.18   $266.20   $266.20  
  Purchase #7 $60.46   5.423%   $63.7387   $63.74   $63.75   $63.70  
  7 purchase totals = $933.25       $1,020.8221   $1,020.83   $1,020.85   $1,020.80