Very rarely. They are usually good at autonomous actions, I can let them handle their needs quite well themselves.
There are some random Sims that have bladder issues if I don't force them to go to the toilet. Yesterday when I was playing my 100 baby challenge one of the kids was so obsessed with playing her violin that she peed herself. For some reason she just couldn't stop playing and go to the toilet. She was playing the violin autonomously, I didn't make her do it, it was completely her own choice to pee in her pants. At some point I noticed that her bladder need was red, but I trusted her to solve the problem herself. I guess she just wasn't smart enough or she's too much into music to care. :D