Abhijit, better safe than sorry. The bears at the Portal rival the Yosemite bears in smarts. They're not aggressive, per se, but they are bold and very cunning. Check out the photos Doug posts regularly in the store of bear-damaged vehicles. Humans are not necessarily a deterrent - these bears have figured out they can bluff people away from their food/packs, then grab and go. They prowl the campgrounds looking for easy pickings, so the closer you are to one of them the higher your risk of being targeted.
It may seem like a lot of questionable effort, but using a locker even for just a few hours is worth it in my opinion. Make sure you include any scented toiletry items as well.
Your best bet in the evening is the lockers in the day use area around the pond. Be cautious about parking in the day use area - Doug has posted recently that the FS has begun ticketing vehicles parked there overnight.
