I had both experiences. I had to disclose to my last perm employer when going to court as I plead guilty, and 8 working days later had been processed through their complete disciplinary procedures to be dismissed for gross misconduct -

I was lucky in that I could move away, and was supported by my wife working. So I went self-employed as an interim manager. My probation officer directly said I only had to disclose if asked and I never was. It was 8 years ago now and from memory I think only 2 of many agencies I registered with asked the ' have you ?', and I just said no, which I felt was a risk worth taking as if they were really serious the company using my services would have done a DBS check, which again, probably because the roles were only temporary never happened. So I would really recommend this route if you have the experience to make this work
Hope this helps