As your hand reading and postflop skills increase, you can loosen up to see more flops. That's why you see the pros and high stakes players entering pots with very marginal hands -- they bank on more advanced skills like recognizing good implied odds
situations and solid postflop play, including a high confidence in their bluffing ability. These are all advanced skills that a beginner or intermediate player often get in trouble with.
The guidelines that suggest super-tight play in cash games are generally geared towards newer players and those who struggle with tough decisions post-flop. You almost never see very advanced cash players being nitty unless they're playing 20+ tables.
Tournaments are another animal entirely -- in a typical fast online tourney, you want to start out pretty tight while turning up the aggression at the appropriate spots and as the levels increase. OTOH in a much slower, deepstacked tourney you can afford to see a lot of cheap flops early on with speculative hands.
So, as with everything else in poker, it's situational. It depends on the game format and structure, your table image, the other players, and skill levels all around.