I have a Yakima rack that was originally bought for my Toyota Tercel
but now can sit on top of either my CR/V or Grand Voyager. Even with
no bikes on it, it barely clears parking garages, and even then I had
to remove part of the tandem rack to make it clear in one parking
garage. Plus, it's a real pain to bikes up and down from such a
height -- I carry a small ladder in the back of the car.
So I'd only go with a top rack if you have a small car, and otherwise
go with a rack on the back of the car. Though that might not work so
well with a tandem. I carry my bike in the back of the car, which is
fine when there are one or two of use, but of course if you're being
energy efficient and carpooling with as many people as possible to
brevets, that might not be an option.
As to remembering that stuff is on top of the car ... just putting the
garage door remote in the back of the car won't help when you're
driving into a parking garage on a long trip. Crunch. Yakima used to
sell a magnetic thing that sat on the hood and blew closed with wind
pressure when driving, but when you slowed down then it popped up. I
wish I'd have bought one, but can't find it anywhere. I tried to make
a homemade version using an aluminum pop can glued to a strip magnet,
but it didn't work well. However, putting that ugly piece of aluminum
glued to a magnet up on the dashboard did help me remember :-)
Even so, eventually one of my family members forgot that we had a
"Rocket Box" on top of the car and drove into a parking garage and
partially crushed it. It's been sitting in the basement for three
years waiting for me to repair it with fiberglass repair tape.