It's worth pointing out that even further down the learning curve (and especially with the expansions!) leeching becomes even more important, but withholding becomes less so.

Leeching is a special side of the more general question of "Pioneer strategy or Leech strategy?"

A "Pioneer" strategy is one where you tend to go in a direction that no one else is going. It has the advantage that other players can't really take much advantage of your actions (so if you're executing well, you'll be able to distance yourself from the pack), but has the disadvantage that other players probably won't be helping you on the side either. For example, in its most basic form, building a Research Labs on the first turn is a Pioneer strategy. Your Explore bonus (keep one extra card) means that you gain a huge advantage from choosing Explore, and other players tend to not want to choose Explore because they'll often be helping you more than you help them. A common result is that you'll end up choosing Explore every turn, and hoping that you can convert that extra income into points by riding on other player's actions.

A "Leech" strategy is one where you tend not to strike out strongly in any direction, but make sure that no matter what the other people choose, you won't be able to fall behind too much. So, for example, if a Pioneer player plops down Galactic Federation, threatening to play Develop repeatedly, you respond by playing Interstellar Bank so that you can at least get a card each time they do so. (Yes, the Interstellar Bank can also be used to augment the Pioneer player's Development strategy.) The advantage here is that the Pioneer player can't pull away from you quickly because he can't execute without dragging you along some ways as well; but the disadvantage is that you're always going to have a close game and so good endgame timing is essential.

As for withholding key cards, it becomes less useful as players get more experienced, because experienced players tend to adopt more fluid strategies that don't have to commit towards a particular direction -- and having albatrosses in your hand that you can't spend can hurt you more than it does me. Sure, if the deck is about to run out, it might be a good idea to withhold until after the reshuffle, but if I'm, say, putting down a lot of Alien windfall worlds, I'm probably already winning no matter whether you withhold the Alien Tech Institute or not.