To Particle or Not to Particle: 今朝は or 今朝


If I'm trying to write: 今朝自転車で仕事へ行きました。

Should I write it as above, or should it be 今朝は、自転車で仕事へ行きました。

Is there a general guide for this? Should I prefer it one way or the other
when speaking versus when writing?

