Proper use of "Using" statement for datacontext

I’m using Linq to Entities and lately, I found that a lot of folks recommending wrapping the datacontext in a using statement like this:

Using(DataContext db = new DataContext) {
    var xx = db.customers;
}

This makes sense. However, I’m not sure how to incorporate this practice in my model. For example: I have an interface (let’s call it customer) and it is implemented by a repository like this:

namespace Models
{
    public class rCustomer : iCustomer
    {

        readonly DataContext db = new DataContext();

        public customer getCustomer(Guid id)
        {
            return db.customers.SingleOrDefault(por => por.id == id);
        }

        public iQueryable<customer> getTopCustomers()
        {
            return db.customers.Take(10);
        }

        //*******************************************
        //more methods using db, including add, update, delete, etc.
        //*******************************************

    }
}

Then, to take the advantage of using, I will need to change the methods to look like this:

namespace Models
{
    public class rCustomer : iCustomer
    {
        public customer getCustomer(Guid id)
        {
            using(DataContext db = new DataContext()) {       
                return db.customers.SingleOrDefault(por => por.id == id);
            } 
        }

        public iQueryable<customer> getTopCustomers()
        {
            using(DataContext db = new DataContext()) {       
                 return db.customers.Take(10);
            } 
        }

        //*******************************************
        //more methods using db
        //*******************************************

    }
}

My question is: the recommendation of using “Using” is really that good? Please take in consideration that this change will be a major one, I have about 25 interfaces/repository c