I only ever do OW. I did obviously ask myself the same questions as you when I first started but I am so glad I decided not to do OWO. Business is always ticking over very nicely indeed.
Apart from the obvious (very real) health reasons I also asked myself (on the other side of the coin) if I wanted a mouthful of rubber as I'd actually never given a blow job in my personal life with a condom so I wasnt sure what it would be like but its absolutely fine, condoms have been much improved over the years and I don't even notice the smell or taste of them. And even if I did - possible smelly willy or rubber? Its a no brainer!!
There are clients who will specifically look for girls who only do OWO and there are just as many clients who look for girls who only do OW. I have been asked when the action has started whether i'll do it without by about 2 clients but I always say no of course and they have been fine with it. 
Ok, I wasnt going to post but I eyed this thread with interest. I do offer owo, but at my discretion. I only now offer this service, after spending ten years as a escort who didnt even kiss, let alone suck a willie unclothed.
I came back to work after a three year break and was horrified by the current market, bb, rimming, owo, etc. etc. But I also noticed something else. Many customers WANT to use a condom. Some do not. Many customers want to kiss, some do not. And so I do the same...I offer the service at my discretion, at a premium, I do it with some, I dont do it with most. It is entirely my call, annd if I was at all unhappy with the hygiene the condom would be out and on before they could say boo. I know I will get replies from ladies who say how could you? Health, hygeine, safety! And I also know that I will strike a silent chord with others who do it and revel in how much easier the booking was.
I am checked monthly, I frequent several 'working girl friendly clinics' and I always ask for mouth swabs. And for those of you still cringing, how many of you have given a unclothed bj to your current squeeze?
*retreats to corner, ready for backlash*