I would say shutting down is completely up to you. I have 3 machines. I turn one of thm off every night, the others havent even rebooted in 3 weeks. The only things with some people is the speculation about processor heat, cooling fans, power supplies, etc. Logically, by design, one would think that turning it off each day after use would make it last longer, but it varies from machine to machine!

sometime,, well... most of times i leave it on for days... but never had any problems

There is no right answer for this one. Matter of preferance. Only right answer is to shut down and unplug power and phone lines when electrical storms imminent, unless protected by very good uninterruptable power supply with excellent surge protection.
Aside from that, I judge you will find opinions on this matter about 50/50. Personally, mine stay on constantly. re-boot every day.

Personally I shut my computers off at bedtime or when I'm not going to be using them for an extended period of time. I've been following this routine since the early 90's and have never experienced a problem.