I used to use Task Managers when I first got my phone but I have since uninstalled them as it has been said numerous times on this forum that Task Killers/Managers are not needed on Android. I think the only time I have missed my Task Killer/Manager was when one of the music apps on my phone wouldn't shutdown right away and I was thinking with a Task Killer I could have pulled it down from the notification bar and killed the app quicker than waiting for the app to respond. That has happened maybe twice in the 4 months or so I have stopped using Task Managers/Killers. Plus Android gives you access to kill running services via Menu Button-> Settings->Applications->Running Services.
As for File Managers CWill is right it's all about your personal preference. I have used many different apps for this and they all have their positives and negatives.