encrypting to untrusted key

[Jacob Perkins]

While working on seahorse development with gpgme, I've noticed that
trying to encrypt text using a recipient with unkown validity fails.  I
assume that this is normal, which is why recipients can be added with a
different validity.  What is the minimum validity necessary to encrypt
text?  Are there other functions that will fail if the associated keys
do not have a sufficient level of trust?  If so, is the minimum that
same for every operation?  Also, assuming this is what the user wants to
do, are there any security risks when adding recipients with a different
validity?
Thanks

-- 
pub  1024D/57DDD9E3 2002-05-24 Jacob (Direct to Home)
<jap1 at rig.dyndns.ws>
Key fingerprint = 1E02 A5FD 8162 4322 C674  6E67 05EB 30F1 57DD D9E3