MB - it depends on how full your flight is, how many people have selected seats in advance etc... sitting together will be based on this, primarily. When traveling alone, I usually do not select seats with F9 and almost always have gotten a stretch select seat.

When traveling with the spouse, I always and I do mean always purchase the upgrade package, we get the seats we want, extra luggage space and check-in and fare refund-ability. My peace of mind, solely.

Anecdotally, twice we had to change flight times or days for a cruise, with the refund aspect I was covered as one time the fare dropped drastically so I could hop on it.

If sitting together matters, get to the airport to check in as early as F9 opens for check-in.

If price is the issue, pay (a little more) at the airport for any additional bags you may have extra.

Safe travels....