If you see a siege weapon, feel free to use it, they’re built in the world for players to use. However, avoid not contributing in WvW and standing near siege hoping the person on it dies (the problem being as soon as you take it over you’ll probably die as well) or leaves. You’ll get as many kills / loot by actively contributing as you would with siege in most scenarios, with less risk (you can not defend yourself on a siege weapon).
Before placing a blueprint, ask on team/map chat if it’s alright and if there is enough supply to build it. Nothing is worse than spending all the time/effort to put a blueprint down and having it rot. This is especially true if there isn’t any supply in the keep that you’re building it at or near.
It’s important to note that placing a siege weapon is a huge undertaking, you’ll need the world to supply “Supply” to it, which is a very valuable resource. There is only 100 Supply in some Keeps/Towers and if it runs dry, then you’ll need to run for guild wars 2 gold. That same Supply can be used to repair doors or for upgrades as well, so you’ll want to communicate with your World before you spend the money to spawn a siege weapon.
Remember that enemies will be attempting to take them out and if you’re up against a siege weapon, you should focus on it before the nearby players as well. They are powerful and take a lot of effort to get setup. Taking one out is like robbing a team of Supply and can remove a lot of pressure from your team.