I have respiratory allergies as well - and when they hit they are miserable! I was given the same info about Zyrtec with my first pregnancy (I'm pregnant with my third) and have now used it as needed throughout all three pregnancies. At times where my allergies were severe I took Zyrtec daily. My first two kids were born happy, healthy, 8 1/2 lbs, past their due dates, etc. etc. They are bright active children at 3 and 21 months, and my current pregnancy (I'm 26 weeks) is going great with another healthy little guy.
I have tried lots of other remedies but honestly none works well for me. I remember reading about what "B" rated means, and being comfortable with it at the time I made the decision. I can't remember now, but I think it had something to do with the extent of testing that had been done, or how long the testing results had been around, or something like that. But there are definitely no known or even suspected dangers with a "B" rated drug.
So that is my experience! Make the decision that feels best to you... I understand where you are coming from, I had one miscarriage before my first healthy baby was born (early, and didn't take anything with that one) so have been ultra cautious with many of the things other people take lightly, from deli meats to gardening.
And congratulations on your new pregnancy - I wish you all the best!!