Oktoberfest

If you do a search, you will find a number of discussions on this, but some short things worth mentioning for your instance:

Is it suggested or necessary to reserve a seat? I did not really want to lock us into a specific time to arrive, but will if it would be a better experience

Reservations are only available for a table of ten, for which you need to purchase several hundred dollars worth of Beer and Food. Even if you were willing, it is almost certain that there are no tables left.

Is there better days to attend then others, or even a better time?

The final weekend will be high times to be around, very active, hard partiers, big crowds. Many think that is fun, evenings are more active as well, it depends if you want rowdy fun or more laid back. Afternoons during the week are still somewhat crowded and fun, but more manageable. Without reservations, you will need to find a free table or seats, this is easier in the afternoon, wander in a tent, look around, ask the waitstaff if there are free seats. You are free to sit at a unreserved table, or a reserved table for later in the day, you just then need to give up your seat when reservation time rolls around.

We will be arriving by train to Munich

Start looking for lodging, finding something in Munich will be difficult and expensive. You might look at towns outside of Munich and train in for the Fest.

Thanks so much. We have reservations now in Erding-so we will be traveling in from there. I am thinking we may opt for the Monday in the afternoon.

Thanks again.

If you can, avoid Saturdays due to the expected crowd levels.
Reservations are in general not necessary. With only two exceptions, all the large tents are required to offer large free seating areas. You can read more on table reservations here.