In Int'l Rumba and Cha Cha, with just a few noted exceptions the weight always arrives over a straight leg. So I'm going to assume you're talking about American style.
With American style Rhythm, there is much debate over how much weight should go to a bent leg. Traditionalists will tell you that you should arrive 100% over a bent leg before beginning to straighten. On the opposite end of the spectrum, teachers trained primarily in Int'l style will tell you to forget about American style technique altogether and just arrive on a straight leg. For the rest of us "moderates", the truth lies somewhere in between.
I teach that the American style bent action is mostly a matter of "presenting", not "weighting" the bent leg. Or at least, not fully weighting it. As the foot moves to its position the leg remains bent, but once the foot arrives, the leg will begin to straighten [i]as the body weight is delivered[/i], so that by the time you are 100% over the foot, the leg is 100% straight. This gives you an authentic bent leg look without actually looking squatty.
There are certain circumstances where I feel that the bent leg action (even the more progressive version I described) should be abandoned altogether. For example, on an Open Break to Underarm Turn to R, or on a Walkaround Turn to R, the step forward & across with the left foot should be fully extended straight. The following step can be done either way, and on the final side step, you return to a normal action. If you try to maintain a bent action throughout the 3 steps (particularly on the first one), it will look like open-legged Lindy Hop.
I also think that the amount of delay you apply to the straightening depends on the dance. I use straighter actions in my Cha Cha and Rumba than I do in Mambo and Merengue, which are the most grounded of all the American Rhythm dances.
The moral of the story: When it comes to American style bent action, everybody's got an opinion. Study the various techniques, then decide what works best for you.
Regards,
Jonathan Atkinson
www.ballroomdancers.com