Two houses....
My house didn't have any sort of bathroom fan. I had one put in when the house was rewired, but it vented to the attic until I had the roof replaced a few years later, it now vents to the outside as it should. I just didn't use the fan so it limited the issues.
Parent's house the fan vented to the attic. Attic had moisture issues, nothing too serious, but they didn't think to address an obvious source of the moisture, ie the fan. I had the fan replaced, and when they did it they also put a new duct in and ran it out an existing roof vent. It's not ideal, but I didn't want to mess up the roof and there wasn't room to get it out the soffit.
If you can easily vent the fan outside, do it. If it's not so easy, then keep it on the list for when something makes it easy. Like a new roof. Don't ignore it though. And if you have problems in the attic because of the moisture, then get it rerouted even if it isn't easy.