I always ask for a front row seat on Spiderman because my 10 year old doesn't like it if he's not right up front when Spiderman "jumps" on the front of the car.
I usually ask for a middle row seat on Jurassic Park so we don't get quite as wet!
As long as you're willing to wait a car or two to get the seat you want, the CM's will make it happen. (Not front row on Hulk or DD, however. There is a completely separate line for that.)
BTTF: This one is a bit tricky. There are 8 seats to a car; two rows of four seats. There is a pre-show in a small room before you board the car. If you are the first ones into the room, go right for the door so you'll be up front and can pick the front row of the car. If you're not near the door, you won't be able to choose your row. As a party of four, if we're entering the car area first, we usually split up two on the left of the car and two on the right as you can enter the car from either side. This way you've got both sides covered.
And, as Barry has already mentioned, if you're in the Single Riders Line, you cannot request a specific seat. You get whatever is available.