It's usually not necessary to specify a channel as most drivers will do a scan looking for stuff (Check your logs, you might be able to see signs of it doing this). However, it can be beneficial under some circumstances to specify a channel. For example, if you've got a neighbor who has a strong signal, specifying the channel might help you connect to your AP faster.
If managed mode is causing trouble, does Auto work? And are you sure it isn't a driver issue?
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