Will constantly having Bluetooth on drain my battery too fast?
What drains... my special. Here some data samples, based on a Motorola Droid:

+--------------------+----------+
| Mode               | Energy   |
+--------------------+----------+
| Bluetooth receive  | 751 mW   |
| Bluetooth send     | 487 mW   |
| Bluetooth standby  |   2,8 mW |
+--------------------+----------+


So if you use BT quite regularly (e.g. to be able to immediately pick up a call with your headset), having it in standby doesn't hurt that much (Compare: 2G StandBy 11,6 mW, 3G StandBy 18,3 mW, Wifi StandBy 7,8 mW -- and, often mistaken: GPS StandBy 0,4 mW).

But if, on the other hand, you use BT once a week (or even less) -- heck, you can manually enable it then, there are enough "toggle switches" and widgets available.