I don't think they intended to give us the option either! They did give us a talking to when we went back the second time and it became clear we'd not done it the first time.
I think, though, because we are a lesbian couple it is rather straightforward. We can't not tell, and we'd have to tackle the 'how come one of you isn't my biological parent/do I have a biological dad I don't know' question whatever we did. So it seems daft to have to be told that, yes, it's considered best practice to tell a child they were conceived by donor.
I guess it might be more complicated if you're planning not to tell them, because as you say, the blood type issue might make it plain later on.
For me the donors' comments do matter, though - not because of us rejecting them, but because I want to be able to look at my child aged 18 and say 'this is why I chose this man - he sounds nice'. Even then, though, it is not a big deal.
We would have loved a ginger donor but they're like hen's teeth (unless you are munro! 
). Hope you get the height you want, though!