Yes, you are correct. You can not file it as an alternate if the control tower is closed, but you can still do an approach into there if you want to if you have communications with ATC. Remember that an alternate is only for when you have lost communications. If you are talking with approach, you can request an approach anywhere if you have to go missed at your destination airport, even if you have an alternate filed you dont have to go to the alternate. When I pick alternates, I always pick one that is very good VFR and is usually a good distance away from my destination. If I have to go missed at my destination because weather is below minimums, I just tell approach I want to either try it again, or go to another near by airport.