I sat my baby up all the time at 5 months She learnt to balance herself and got stronger. Then she would fall and over the course of two weeks learned to push herself up into sitting position on her own. Crawling came next - at 6 months. Then at 6.5 months she started pulling herself up into standing position using the furniture! No harm sitting bubs up. Getting their balance just sitting helps them with their core strength. Let your baby lead the way and just trust your gut instinct. All babies are different. Now she’s 7 months (in three days) and I’m positive she will be walking early! My first baby was so tubby and heavy he couldn’t even sit until seven months lol - but he was walking by 1 year bday.
My son was sitting unsupported around 6.5 months but still can't get there himself. I was just thinking about the crib position yesterday because it's still on the highest setting. He's not pulling to stand yet either so I guess we've got a little bit of time but it should be done sooner rather than later.