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 $239.92   13.873%   $273.2041   $273.20   $273.20   $273.20  
  Purchase #2 $87.65   10.514%   $96.8655   $96.87   $96.85   $96.90  
  Purchase #3 $211.85   7.599%   $227.9485   $227.95   $227.95   $227.90  
  Purchase #4 $100.47   4.410%   $104.9007   $104.90   $104.90   $104.90  
  Purchase #5 $163.88   13.194%   $185.5023   $185.50   $185.50   $185.50  
  Purchase #6 $128.57   9.209%   $140.41   $140.41   $140.40   $140.40  
  Purchase #7 $219.36   4.445%   $229.1106   $229.11   $229.10   $229.10  
  7 purchase totals = $1,151.70       $1,257.9417   $1,257.94   $1,257.90   $1,257.90