my clients are more than happy to keep their dogs away from any of my girls who aren't comfortable with them. You are only in their for a few hours so it's not like it's gonna be locked away for ages, eh?
You are providing them with a service, they feel it is only right that you are comfortable in your surrounding whilst you work and aren't looking over your shoulder every 5 mins. Besides they also realise you are more likely to do a good and thorough job if you aren't distracted by the thought of their pet leaping on you at any minute!
Having said that I would still be inclined, if you can bear to, to introduce yourself to the woofer first!
BTW well done and good luck!
Jan