If you consider your US address permanent, then there shouldn’t be problem. Nobody has to notify their broker every time they go on extended stays abroad.
At any rate, even if you did consider Taiwan your new, permanent home, they would most likely prefer to not know. If you’re willing to take some of their mailings at the US address, it means cheaper postage for them; but most of all, it keeps it simple. A US citizen living abroad who would prefer to use their foreign address as the main address is a paperwork/data-entry oddball, as Stragbasher pointed out. I’ve run into at least two brokers that sign up non-US citizens living outside the US, but for some reason have nothing for US citizens that are long-term expatriates. We’re just way off their radar.
Of course if its an option, using an available US address is the smart way to go. I originally preferred to not use a US address because the one person I trust is paradoxically someone I would prefer remain uninformed about my financial status. The all-web-based statements makes this a moot concern, but it’s still annoying that I can’t be honest – that I still have to play these “don’t ask, don’t tell” games because of bureaucratic sloth, especially when I’m the customer.
In the end, though, I gave up. I decided that annoyance was preferable to having to deal with complicated sign up procedures or higher commissions, and went with the one offering the lowest commissions using my US address. During the next few months, I’ll try to shift the mailing address, and then the permanent address, to Taiwan. Should be better to throw curveballs after becoming a customer.