Hi Petr,
The answer to your question is Yes and No. You are
looking for a simple answer to a complicated question.
The following site
(http://www.maxim-ic.com/appnotes.cfm/appnote_number/279)
has a great article entitled "Supervisory Circuits Keep
Your Microprocessor Under Control" which sheds light on
this subject.

This same question has been asked many times many ways
in the past. It brings two types of responses. One crowd
says you need the supervisor IC and the other crowd says
you don't. Most of it is based on their own experiences.

Hope this helps.

Regards,

Charles Bannister