If you have any version of premiere pro, working with YV12 clips (ULY0) is mighty fine. Be advised that some effects are YUV and other effects are RGB. CS5, afaik, will tell you which colorspace every effect works in. If you are using a version of premiere before pro, then it's safer to convert to RGB24 or RGB32 (you only really need to do 32 if you have alpha). Do keep in mind that YUV clips will be smaller than RGB clips, with YV12 being the smallest.
And yes, x264 requires YV12 input (well, it technically doesn't anymore, since it can internally convert the colourspace now, but still...), so if you keep everything in YV12 from start to end, it will be best.
PS: Since DVDs need to be MPEG-2, they are all YV12. Blu-ray discs can have more codecs, with them being H.264, VC-1, and MPEG-2. If it's H.264 or MPEG-2, it's pretty sure to be YV12 (could technically be other YUV, but don't think I've ever seen it myself). I don't know much about VC-1, I would assume that one is just YV12 as well, but don't count on it.