Depends how the company is doing. Depending on the internal transparency of the company, as an employee you're often privy to a lot of information that Wall Street does not have access to, eg. you'll oftentimes know their upcoming product pipeline, employee morale, culture, and key metrics for the success of the business that are not published in their financials. If those are doing well but Wall Street is treating the stock poorly, it makes sense to max out your stock compensation and never sell, at least until the metrics turn or you leave the company. (Yes, technically this constitutes insider trading, but it's virtually impossible to prove that inaction was because of inside information vs. because of lazyness or inattention, unless there's a pattern of action.)
As an employee at a megacorp, sure, you can see a little bit about how things are going from the inside. That said, you probably spend less time looking at reports than professional analysts. Secondly, sure, maybe you can see how great your company is, but do you have a frame of reference to a million other companies and their projects and cultures? There's a strong bias towards thinking you know more than the market that I would be wary of.
That depends a lot on your past background. A number of the employees at these megacorps have either worked in finance before, or at one of the big consulting firms, or they've done the Valley dance between the hot growth companies of the moment. So they do have that reference point between many different companies. There's sort of a revolving door among many of the elite institutions that run the world - Ivy League universities, large dominant corporations, big-name consulting, finance, and government.
If this doesn't apply to you, and a megacorp was your first job out of college that you stuck with for your whole career, you may want to be a bit more cautious with your capital.
That saying's usually not true when you're dealing with employee stock compensation, where you own the shares outright and make enough in cash salary to live on. Ownership is forever (modulo a revolution or other forcible overthrow of the rule of law, in which case you have bigger problems). In the absence of leverage, you can afford to be perfectly rational and have an infinite time horizon.
The company I used to work for not only didn't provide stock options or RSUs or (that I can recall) an ESPP for ordinary employees, they eventually took the choice to invest in company stock away from the 401k plan, because they decided it would encourage people to not diversify and they might be considered liable.
I remember when the CEO visited, and while she clearly didn't know that our division existed, the things she talked about highlighted how little we knew of the rest of the company. You have something with tens of thousands of employees, and you have probably quite a few groups of a few hundred people that just have no particular connection to the rest of the company. In our case, we started as an acquisition that was kind of forgotten about.
