In my opinion, it’s better to do the SS office and not wait on the 485. You need the social security number for certain things, and the sooner you have it, the better.
The only thing is that with the K1 visa you need to get it within 74 days of arriving, but it’s advised to not go before 14 days after arrival. This is because the systems don’t update often and her arrival/documents might not be in there and they’ll have to ask USCIS questions, which takes longer. 74 because they can’t issue a card after your I-94 expires within 16 days. It’s a hassle.
For me personally, they had to investigate if I could even get a SSN, this took 3 weeks, then my card was supposed to come first week of December, never came, and with all the holidays etc they couldn’t issue me a new card. I do have my number, just not a card. It’s drama. But I’m one of the unlucky cases.
Only thing is, if you get one yourself, you have to have it when you file 485, because it asks if you already have a SSN, and if you do, you can’t lie or leave it blank.