The short answer to your question is, it depends. Because traffic is so challenging in the DC Metro area both the 10am and 7pm time slots seem best to me to give participants a fighting chance to get to an event.

However, depending on the demographic of the networking group you may want to tailor the timing to either during work hours if appropriate or after work hours for that group to best suit them. You may also need to take into consideration if most participants have young children, the timing may have to be scheduled around parent/ children schedules.