Did you miss out some "else"'s ? Would this work:

if (endTime == defaultDate) endTime = DateTime.MaxValue;
>sqlStatement.AppendLine("salespoint = @SalesPoint AND date_time between @StartTime and @EndTime");
>sqlCommand.Parameters.Add("@StartTime", SqlDbType.DateTime).Value = startTime;
>sqlCommand.Parameters.Add("@EndTime", SqlDbType.DateTime).Value = endTime;

or will it be less efficient on the SQL side ?

I think it may be as efficient as the original, I like it - I am going to adjust my code to use this version instead.