Inside the iPhone Reset Settings, you have a number of different options. The one at the top is Reset All Settings and should only be used if you are positive about your actions. If you feel as though you are going to lose something that you may want do not Reset All Settings, but instead double check all the features.
The more serious option is Reset All Content and Settings. This option should be the absolute last resort as many people have important photos, sound media, video media, text content, contacts, emails and many other items that they may not want to lose.
The block below this is to Reset Network Settings and is a good idea if you are leaving an area where you use networks you are not going to return to. Do not do this if you are remaining fairly static in your general locations. The third block has Reset Keyboard Dictionary, and since you will be building this up constantly there are very few situations that this is going to benefit you. The times that it would benefit you are also the times that you would probably do a clean reset sweep on your iPhone anyway.
The Reset Home Screen Layout is also one that is not going to be of much use, but it can if you simply want to start over on a home screen construction. This is a good way to get all the pre-installed items onto your home screen. The last one is to Reset Location Warnings and this should just be avoided, as it will have almost no benefit outside of a clean sweep situation.