The point is to get into the habit of giving efficiently. Going through the process of choosing which effective charity to give to is a distinct skill from choosing to give in the first place or acquiring the jobs that will give you more money later. So it's worth practicing. And it's worth conditioning yourself to take pride in that effective-charity-selection process.

I don't think most people actually "stock up" on fuzzies. Donating to fuzzy charities may simply build the habit of donating to fuzzy charities and then make you feel bad later when you have to switch. (One or both of us may be generalizing from one example here). You'll still want fuzzies when you're older (if you care about fuzzies at all), but you should, at all ages/incomes, practice acquiring fuzzies through the cheapest manner possible. (This probably varies from person to person)

There are good reasons not to donate a lot of money when you're a student or still searching for good jobs, but getting into the practice of thinking about charity rationally is still important.