Choir and string sounds often sound too large or too small to me when it comes to samples. What I mean by this is that it either sounds very large - like a whole orchestra string ensemble or a large choir (which often just sounds too big to be useable), or it sounds small in an unrealistic sort of way - usually where the realism of the sample is only convincing in a certain range of notes.
What I want is a believable, simple, small-room, close-mic'd group of voices or strings that sound like a string quartet or caroling group. I want the ability to call this up in my EMU 4XTultra sampler and play these sounds whenever I need a believable string or choir group. The best way to get these sounds is to not dig through library after library to find the sound that you want - but to sing or play the instruments yourself or get a few friends over to sing or play a few scales of notes, sample these into your instrument and map/spread them across the keyboard so they are playable. You might ask "Why not just perform the parts separately (alto, tenor, etc) and mix them together?". While this is possible to do, sometimes you just want to quickly try different ideas and see what types of sound work best in your song.
When doing this, I usually record the raw session into a track on Protools, then use my Conn Strobotuner to see how close the pitches are. If they are flat or sharp, I will shift them a few cents to compensate, but making this too perfect or using autotune will suck the life out of any convincing realism; so it is important to leave some pitch fluctuation or nuance in the recording. Once this is done, it is easy to add a compressor or EQ to get the sound exactly where you want it to be. I don't like to use echos, delays, or reverbs before I sample the instruments because it is much more flexible to add them after they are sampled where you can tailor the sound and mood to the song you are working with. Sampling every chromatic note in the scale is ideal, but only if the instrument or voice was played or sung for similar note durations, with similar vowel, vibrato, and tone.
Typically, when I have a section of strings, choir, horns, etc., I record a track of samples first to see if I like the way things fit together, then I will record some tracks of real instruments on top of these where the slides between notes, nuances of playing the same note, and subtle variations are captured to make the entire group of instruments sound real and convincing.