I'm commissioning in december. I'm not sure about service selection, but as far as the score used to determine your class rank during ROTC, there is a slight weight given to your score if you are a technical major. Thus, a higher class rank, which, as far as I know, helps in service selection.
But the boost is small. Someone with a 3.0 in underwater basket weaving will probably still end up better than a 2.5 in engineering.
THAT being said. Only take a technical major if you really like it or want it for some reason...test pilot school, or something that requires it later on. You'll be must better off taking something you remotely enjoy and not struggling through 4 years of engineering... if you don't think you can pull off a 3.0 in engineering, don't do it.