If I buy a full reel of components (let's say resistors) with the intention of sending it to an assembler later so they can assemble my product, is it OK to use components from the reel when I'm soldering at home, or will this make it difficult/more expensive if I later send that partially used reel to the assembler?
The assembler is going to have to pull some of the tape from the reel regardless, so any components that are under the end of the tape will likely be wasted anyways. Using them for your own purposes instead of letting them fall on the floor should not be a problem. Just make sure to not cut off any of the reel or tape so that they don't need to pull off even more tape.
Full reels come with 2-3 turns of extra cover tape at the beginning of the reel. If you want to harvest some parts do not cut this extra cover otherwise assembly house would have to make it, wasting parts in the process.
You might contact your specific board house with that question but I speculate that there would not be an issue so long as:
- You follow any ESD precautions required for the particular components when handling the reel and
- You don't confuse any inventory tracking that is in place for large numbers of expensive components.