Using dapper.net and repository pattern in Azure websites

I am pretty new to dapper.net and i would like to have some feedback on my current implementation.

I'm currently on Azure Websites & Azure SQL, some documentation recommends using Async as much as possible as cloud services higher latency and have chances of dropping traffic, but it's also cautioning that Async/Await have overhead.

Should I...

1. Implement an IStoreRepository interface (although I don't know if it's helpful to me)?
2. Use Async versions of Query and OpenConnection?
3. Am I repeating myself so much in different Repository classes like GetById, GetAll, etc? Is there a more elegant solution?

StoreRepository

public class StoreRepository
{
    public async Task<Store> GetById(int id)
    {
        using (var conn = await SqlHelper.GetOpenConnectionAsync())
        {
            const string sql = "Select * from Stores where Id = @Id";
            var res = await conn.QueryAsync<Store>(sql, new { Id = id });
            return res.FirstOrDefault();
        }
    }
    ...
}

StoreController

public async Task<ActionResult> Details(int id)
{
    var db = new StoreRepository();
    var store =  await db.GetById(id);
    return View(store);
}

SQLHelper

internal static class SqlHelper
{
    public static readonly string connectionString = ConfigurationManager.ConnectionStrings["ttcn"].ConnectionString;

    public async static Task<DbConnection> GetOpenConnectionAsync()
    {
        return new ProfiledDbConnection(await GetOpenConnection(false), MiniProfiler.Current);
    }
    private async static Task<SqlConnection> GetOpenConnection(bool mars = false)
    {
        var cs = connectionString;
        if (mars)
        {
            SqlConnectionStringBuilder scsb = new SqlConnectionStringBuilder(cs);
            scsb.MultipleActiveResultSets = true;
            cs = scsb.ConnectionString;
        }
        var connection = new SqlConnection(cs);
        await connection.OpenAsync();
        return connection;
    }
}