My typical habit is to press the lock button on the inside as a step out. This works just fine, except when my daughter is in the truck with me. For whatever reason, she always to be several seconds behind in exiting the truck, and doesn't even have her door open when I am ready to lock the truck. I find myself waiting on her so I don't lock her in. I know I can lock by keyfob afterwards, but I just don't want to reach into my pant pocket, or think about having to lock the truck. Little annoyance.
Locking the truck, or locking anything for that matter...is sort of a weird exercise, IMO. First, there is no reason to lock a truck if there is no thief to steal the truck or it's contents. Since the majority of people are not going to steal due to personal morality or the deterrent of legal punishment, the risk is kind of low.
Second, a locked truck and unlocked truck look identical. A thief isn't going to look for unlocked truck, testing every door. He will assume all targets are locked and he's going to have to break in somehow. Sure, unlocked truck is easier, but he won't choose the target based on whether or not it's locked. It's somewhat similar to herd immunity, in that you don't really need to lock your truck as long as the vast majority of people are locking their truck. So basically, leaving your truck unlocked isn't going to significantly increase your risk of theft. I would argue that it more important to get a vehicle who locks are not easily bypassed than it is to habitually keep your truck locked.
That said, I do habitually locked the truck. What I try not to do is freak out about whether or not I remembered to lock the truck.