Originally Posted by ADB25
As PIC, you do what ever you feel you need to for safe flying practices.
Simple and most definitive answer. If you think there is an aircraft in the vicinity of your planned flight path and the person "responsible" for passing you that information hasn't done so, it is not only smart, but I'd say it's actually neccessary for you to get the required information - unless of course you have the ability to spot every possible traffic in every situation or you blindly believe the Big Sky theory... If the traffic is near, or coming in opposite, it may not be enough time to get adequate information if you first ask ATC/FIS/AFIS/.... which first acknowledges, then asks the other aircraft involved, wait for a response, ...
ICAO Annex 2, 2.3.1 Responsibility of pilot-in-command
The pilot-in-command of an aircraft shall, whether manipulating
the controls or not, be responsible for the operation of
the aircraft in accordance with the rules of the air, except that
the pilot-in-command may depart from these rules in circumstances
that render such departure absolutely necessary in the
interests of safety.
Full stop.