Normally, I'd agree with stevez that just blowing it out is the right thing to do. However, as I think about it, I might vary my response based on the amount of snot that needs to be expelled. For instance, a runny nose just requires a little dab and a short snort; if, on the other hand, I feel serious mucous coming on, I might stand and walk away from the table before letting it rip. Standing up also has the advantage of straightening the wind pipe for more expedient explusion of mucoid material; in addition, in a standing posture you are more able to fill the lungs with the right volume of air required to quickly and efficiently evacuate the rheum.
With all this in mind, during flu season, I'd probably bring my own Kleenex and waterless antibacterial soap when going out for pho.
Hammond
"Don't you ever underestimate the power of a female." Bootsy Collins