That depends on the pattern. If the pattern strinks (or rolls, etc) a lot, then I pin it (with rust proof pins). If no shrinkage (or rolling, etc), then I don’t pin it. I also put a towel or pillow (depending on the pattern) on top of the pinned work. If it’s stockinette, I normall put a pillow on top of it too. But if it’s cable, I put a towel so as not to flatten the cable out of form.
Anyhoo - for sweaters, I use the floor or “coffee” table if it can fit. Or I block each piece to measurement separately on the ironing board. Then seam the pieces together. Of course this only works if the pattern gives you the measurements of each individual piece and if the sweater is even made in pieces