Finding out Negative Keywords along with Positive

[motionz]

Is it really important to determine the negative keywords along with the positive ones for my PPC campaign? I mean finding out only the positive ones wouldn't be enough?

Some PPC managers suggests it to do it because there is a very important element in this that could lower your conversions. Suppose, if your site content is about 'gardening equipments' then obviously the word 'cars' will be an irrelevant one. But if you put keywords such as 'backyard gardening' or say 'ceremonial gardening', then may be they are related or relevant to your original keyword but they will not get you conversions but instead of spending a lot of money in PPC without getting any results.
And in PPC those keywords which cannot get conversions are termed as negative, so finding out negative keywords are also important.