Check on all your potential schools. If I'm not mistaken, the ACT web site even has a chart. (If not, or something isn't listed, go to the colleges' web sites.) For some places, the writing part isn't optional!
If you think you might take the test more than once, you could do it with the writing section and see how you do. If you don't do well and don't need the writing section, take the test again without. ACT has score choice, so colleges need not see the first scores.