Hi @secilia,
Great question! Choosing whether or not to cast a circle is up to the caster - some may always use a circle, while others use them depending on whether or not the spell calls for one, while yet other witches choose not to use circles at all.
I do want to echo what others have said: if a circle is opened, it must be closed when the spell is done. It’s not dangerous per say (depending on what type of spell was being cast and any entities involved), but leaving a circle that was intentionally set open can disrupt the energy in the space and lead to messy spell results.
There’s a helpful guide here with more information, as well as some basic ways to open and close magickal circles - Tracy kindly shared it above!
If you’re interested in reading some experiences with people using circles, we did a circle challenge a while back (you can scroll down to read the entries in the comments) :
Whether you choose to use circles or not, happy casting and blessed be! 