small buds and only leaves if they are coated in trichomes and probably only for ice-water hash at that. A sieve won't be contaminated by it and you get the trichomes off.
leaves - 1/100th the thc.. stems 1/1000th thc of buds
On the other hand, tiny buds and/or larfy buds are great for extractions.
It depends a bit on the extraction method. e.g. i wouldn't use stems for ice water hash or rosin, but if the container is big enough and you don't mind using more solvent (cost-effective?), then go ahead and toss in. Really depends on convenience and cost as the return from stems and most leaves will be incredibly low.
The thing with solvents typically used is they are not too selective at what goes into solution. All the oils, not just thc and such, will dissolve into it. so, adding less pure plant material will add more contamination, i bet. comparing buds of 20% of mass is what you want vs ".01%" of mass in stems. (i.e. some small value of irrelevance)