I have a circuit that uses a PNP transistor to enable or not the current to flow and turn on a LED. The point is, is it a good engineering practice to conect the base of the transistor to the Vcc of my NE555P, so that it will be grounded through the ~15kOhm resistance of the IC and saturated whenever there is no voltage in it, and when there is 12V in Vcc it will open the transistor as I desire? Another solution would be using 2 NPN transistors in a logic that would do the same job.
No, it's a terrible idea. There's more than just the three 5k resistors connected between Vcc and Gnd inside the 555. In fact, the PNP B-E junction will pass enough current to operate the 555, so it will never actually turn off.
You could do it with a P-channel MOSFET, however.