On Tue, 30 Oct 2012 05:04:40 -0700 (PDT), wrote:

I bought an Iphone 4 (on O2) a while back and am having problems with coverage, often have no coverage showing, in settings it was set to Automatic for the network, Is it a good idea to change this to o2? if not, why not?

Have it set on your contracted network (O2) for if you don't, and it
keeps connecting via other networks you'll get charged for roaming.
Roaming is fine if that's what you want and don't mind the costs.