Does anyone know if check-in time affects seat assignment? I know with some other carriers they give away the most favorable seats last (if available).

While that may happen on occasional, there is no such UA policy and generally the most favorable seats are gone by checkin due to high load factors.
It is true BE can sometimes get a middle E+ seat (which is only better than a middle E- seat) and few would call it most favorable.
UAs goal is to fill every seat (and for most domestic flights, IME, it succeeds.

BE seating is not just assigned at checkin, may be earlier or later.

not that I would but BE myself, but if you want the most favorable seats, either buy a fare that includes seating, or if you buy BE, pay for a seat assignment in advance.

maybe you�ll get a good seat if you let UA pick it for you, but there�s, at best, equal or more likely less than that of getting a good seat last minute. Those who pay or those who buy a fare that includes it are going to pick the more desirable seats first otherwise, and sure, someone might cancel, etc., but if the goal specifically is a better seat, use one of the most guaranteed ways to get it.

put another way, if your goal is to simply save the most money - buy BE and you�ll get the luck of the draw on seats - possibly a good or ok one, or very likely not (I�d honestly assume the latter, and be happy with anything better). If your goal is to get a good seat, your best bet is at time of booking. If you�re straddling between saving money and getting a decent seat, buying BE and a seat assignment at booking is probably your best bet.

In my experience, no, check in time does not seem to matter for basic economy on united

Good to know that the assignment can happen at any time. It is for a EWR-MUC flight and the inverse. I bought BE to keep it cheap, I have the Chase card for bags. I�d be happy with anything besides a middle seat at the back but if I end up there it would be fine considering the savings.

My personal experience with BE fares is that the seat gets assigned automatically at T-24, regardless of when you check in. You can see your seat assignment appear by going to your reservation online. As far as I can tell, they assign all BE pax to middle seats at that time, or at least as many as possible. I have never not gotten an E- middle seat on a BE fare, though I haven't bought too many of them for this reason. Now I only buy them for short hops where the BE discount is significant and I don't care that I'll be in a middle seat in the back.

One the rare occasion i have purchased a United BE seat (and assigned middle in the back), I have approached the gate agent and politely asked if there was a better seat available for a Gold million Miler and have generally been accommodated if there was one available. Makes sense to me. The plane is about to board and if they haven't sold the better seat by then, they never will. A little goodwill for the pax and no loss of revenue for UA. And United needs all the goodwill they can muster these days given their atrocious performance over the past few months.

Ah, the new version of "The Speech"

That information is good to know. I�m going to try BE out this week; I�ll see if I can get a better seat.

In the rare cases of being a BE fare I have always gotten a nice E+ seat at the gate

I'm going to take a wild guess . . Oktoberfest?

Much less fun, going to the U.S. to put years of personal items in storage. I�m just passing through Munich airport starting from Italy. I listed the return red-eye first because that�s really the only one I�m apprehensive about.