If you use the water/butter method (which is required for potent butter from weak weed) then you will need to put the wet weed/butter sludge in to a cheese cloth and squeeze really hard to separate the oil from the weed. If there re stems in it they will rip the cheese cloth. Also the stems being more stiff will soak up oil and be really hard to strain. If you use a steel press then the stems will make fibers that jam things up.

I think for stems you would need to use all stems, or maybe make hash oil with a solvent like alcohol.

Long story short, you may be able to use them, but it will be a big hassle for a little bit of TCH, and you may end up with less butter than with no stems at all. If I had not smoked weed in months and had no chance of getting more good weed, and I had a huge pile stems, I would try a big batch of stem butter.