Bending it down does mean signal losses, the question is how much loss is acceptable. A quarter wave vertical (9 foot) with good grounding and nothing around it is as optimal an antenna as you'll get.
If you stuck a clothes hanger into the back of the CB, you'll get some signal out into the air, and you'll be able to talk to someone before the magic smoke starts coming out of the radio.
Everything in between is a compromise, with power converted into heat instead of being transmitted, or power reflected back into the radio. The coils are a compromise and have effects like diminished range, and smaller bandwidth (more signal loss on the upper and lower channels than you would normally get).
The k-40 and Wilson 1000 have coils at the bottom, firesticks (essentially a coil the entire length) are also great but not as much with a magnet mount when you're going fast and all of a sudden hear a whump as it comes loose off the top of your car. To get out well with a coil, you just need to make sure the coil is above your car. So on the roof is good, the trunk not so much.