For me, it's an absolute MUST DISCLOSE, although I don't think too many people operate this way. I wouldn't NOT marry someone who had scores at the bottom of the barrel (mine were!) but I would not marry them until we had come to a decision & had a game plan on who would handle finances, how we would correct the problem. To me, having a crappy CR isn't a sign of bad character. Being unwilling to disclose it, unwilling to take responsibility for why/how you did it, or unwilling to commit to change IS a sign of bad character.
My first "marriage" (I say "marriage" because we had a ceremony but he wouldn't get a marriage certificate because he was afraid of the government tracking him.. THAT WAS A SIGN, IVY!!) was a complete mess. We met when I was 18 and he instantly started "guiding" me and getting me credit, to "build my profile." It never occurred to me that he was using my SSN & CR because his was practically in the negative numbers. By the time I left him 4 yrs later, aside from having two children at the age of 22 and more than 17 hospital bills on my CR from the abuse, my score was a 424. There were numerous cell phone bills that I didn't even know about, a SL on the verge of default, and he had run up & done cash advances on my CC at least three times w/o my consent. The bal on my CC was $2865 with a CL of only $1100.
When I met someone GREAT a while later, I made both of us sit down & look at each other's CRs. His wasn't horrible, but wasn't great. He's got a high level clearance in the military, so he couldn't afford to totally blow his credit, but he had a track record of late payments, charge-offs on small am't CCs, and took out $1K-$3K personal loans from his bank at least once a year. So, when he asked me to marry him, we decided that *I* would handle all finances and he would only get an allowance. That's how it's been ever since. Now, almost three years later, we have $0 old/bad debt, two cars at mid-int rates (compared to his 18.99% APR that sometimes went as high as 26%) and he has NO allotments coming out of his pay.
I would not have married him without full disclosure & without commitment to a plan to change. I would have expected him to not marry me if I hadn't returned the favor, but, he's much more the romantic type so I know he would have. He was actually very relieved that I wanted to take over the money & lead/teach us both in that department. He knew he wasn't very good at it, but didn't know how to change.