If there's a radar service, and you're in IMC, using it is a good idea. It's not a bad idea in VMC either. I often get VFR traffic pointed out to me by LARS units. You can look and look and still not see. It's the low-down dark-coloured ones I have trouble spotting.
But I've flown (and been flown) many times under IFR in IMC in open FIR, with no radar or anything else, and wasn't worried. It's a big sky! Just avoid the honeypots.