Does he say anything about what you should set to power with Helios One? If not, what do you guys think he'd prefer me to power? It's hard to decide, I just know that he wouldn't want me to use it as a weapon.

Cant remember what his opinion is but im sure he would want it sent to power Vegas. Seeing as how he wants the electricity from the dam to go there as well

Depends on what you'd want to do for Vegas. House probably wouldn't care so long as the power supplies Vegas. From an ideological perspective, I power Freeside. When House wins Hoover Dam, The Strip will be powered still, but Freeside, which is a part of Vegas, needs power direly. From a 'Good of the Realm' perspective, I'd say reroute power to Freeside.

He most likely wouldn't want it set to the weapon unless he was sure that only he could control it. Then it would give some extra punch to his military might. A sort of ace in the hole. As it stands however, if that C-Finder gets in the wrong hands, one shot in the middle of the Strip could cause a hell of a lot of damage.

He'd most likely like it to be sent straight to Vegas. Or possibly to Fremont and West-Side, as that would satisfy some complaints in those areas. According to House, Hoover Dam provides more than enough power to Vegas itself. So he probably wouldn't care in all likely-hood.

So, it sounds like Freeside would be the best tactic. New Vegas has plenty of power, that's certain. Even without Hoover Dam.
Freeside has little to no power at all, so powering it would be extremely beneficial to not only House and the Courier (since anything that benefits House, benefits the Courier in turn, given that he's House's protege and likely to replace him when/if he dies, assuming the Courier wasn't responsible for it) but to everyone in Vegas, and possibly the Mojave itself (since the Followers of the Apocalypse are based in Freeside, and could expand)