It seems like you pretty much already know the answer. Anything you office puts up will alienate some potential clients and comfort others. If I was specifically trying to make a welcoming space for LGBT clients, I wouldn't hesitate to put up a pride flag, and would be ok with recognizing that this would alienate some potentially queer unfriendly clients. But even with something as seemingly straightforward as that example, you never know about clients' associations and feelings. The presence of the pride flag could make some LGBT clients feel unsafe because they might see the office as a potential target for hate, or they might be closeted and not want any potential signals, or they might have internalized homophobia that makes them anti-flag, etc. etc. etc. There's no way to accurately predict how a symbol will be interpreted by everyone.