I personally would hope that if I was to ever be put in the position where I lost my job, I would remain on good enough terms with the company to not do that..

I think it very much depends on circumstances, and I’m sure we’d each be able to make the appropriate call based on those.

In a situation where you’ve lost your job because the entire company has failed, your employer’s old clients would suddenly be without a designer and may well be very pleased to hear from you. Continuity can count for a lot in design.