Pretty much, as long as they control either the domain account, or the nameservers they *could* screw you.
You can transfer the domain away from them, but unless you sign up for DNS service somewhere they can still point your site to /dev/null or a p o r n site, if they so choose.
Do you trust them? Do you owe them money? Why would they screw you?
edit: it's just a matter of how paranoid you are?
PS: did you see someone over there?