IFR, in case you did not realise, means "Instrument Flight Rules". This means that no, for most of the flight there is no requirement to look out of the window.
Shurely Shome Mishtake?!?!
A French accident (or maybe Airprox) report that came out over the last few weeks specifically blamed the IFR crew for not keeping a look-out in IFR in Class E airspace.
Of course the IFR pilot/crew is obliged to use all possible means to avoid a collision and looking out of the window can only help!
Timothy