From a military helicopter perspective - it's mostly cultural unless a tactical need exists. Even the tactical need is debatable depending on a variety of factors as far as I'm concerned.
As Nosehair said, there are some possible mechanical emergencies that would make being close to the ground more desirable. Some that come to mind that are at least plausible are any kind of fire... and, that's about it. In the Chinook an aft transmission hot light means your aft transmission could easily catch on fire so I'd like to be low for that one too. Maybe a degradation of tail-rotor thrust too as it is very likely you could lose your tail rotor altogether. A complete tail rotor failure is going to require an autorotation and personally I'd like to be at altitude for an auto so I have a better chance of picking a good landing area... same goes for a dual engine failure (or even just a single engine failure if it's hot and I'm heavy with non-jettisonable cargo.)
Overall though, the slight possibility of one or two mechanical emergencies doesn't really see the argument to fly low when other mechanical emergencies would be handled better up high.
When I got back into helicopter flying after fixed wing flying I asked around about why pilots were selecting what altitude... rarely if ever did I get solid reasoning. I mean, unless an operational need exists such as doing rescue work, aerial firefighting, logging, tactical ingress/egress, etc, why fly at 500 agl? We fly IFR all the time at normal IFR altitudes above 3000 feet.
Tactically there are a significant number of reasons to fly higher. My first tour in both Afghanistan and Iraq saw the most common altitude at 50 ft agl. There were some okay reasons for doing so, but these days I think it makes sense to go higher. Without going into classified detail, our missile warning and defense systems work better at higher altitudes. Also, small arms and RPGs are far less effective at higher altitudes. Plus, when it's 120 degrees on the deck getting up a few thousand certainly feels better.
That being said, I'll completely contradict myself by saying in the States while VFR I generally fly at 500 - 1000 agl. The primary reason is because I'm lazy. In the 2000 and up range there is enough fixed wing traffic to make me feel I have to get flight following and the nature of our missions makes one more person to talk to that much more busy in the cockpit. A secondary reason is because of primacy. I was born and raised a helicopter pilot and you only go high when IFR. A tertiary reason is because it's fun. The view is a heck of a lot better at the AGLs than it is in the MSLs - and forget about the FLs. Heck, 500 agl seems high going point to point when in tactical training areas I'm down to 50 agl weaving through hills and valleys.
So even though I flip flopped on my own point, my saving grace is that at least I have a reason for whatever altitude I pick, no matter if it's high or low.
