Sounds like a pretty good idea, especially if your commute puts you in a mix with truck traffic. Remember that drivers of big rigs, as well as some urban trucks & vehicles (ie. garbage trucks, delivery vans, buses, etc.) sit up much higher off the ground than car drivers. Having a light higher up (in addition to your seatpost or rear rack) might help them see you better. It probably helps with long-distance visibility a bit too.