I haven't had to sub out yet (almost did, but didn't), but I did just update my T+C's to include a short statement about the potential use of subs if needed. I said something about how my clients' info would not be given out, etc. - & how any sub I choose to work with will be accountable to me, & therefore any agreements & assignments would be established between us (not them & the client).
I thought I was going to have some phone work to do which I'm not interested in, but have someone who doesn't mind it - in the end though, I changed my mind about working with that client on that task for many reasons, but it still prompted me to add that statement to my terms just in case it ever comes up again. Now my clients (& potential clients) will know my intentions. I'm not sure if I would tell them on the fly if I was using a sub to help with a project, I just think it's good practice to let them know there could be a possibility though.
Of course, I do also agree with Tess about the level of skill or task that's being provided.... if it's more involved like web design or something similar, then it's only fair the client understands who's doing all the work - however, I still think I would keep the flow of work going through me anyway to hopefully avoid confusion (& still keep the accountability issue between me & the sub).
Just my take on it - I hope I made sense... lol