As far as I know, in C# all fields are private for default, if not marked otherwise.
class Foo
{
private string bar;
}
class Foo
{
string bar;
}
I think for readability it is always best to be explicit.
As a side, You may wish to have a look at a visual studio plug-in called Code Style Enforcer (http://joel.fjorden.se/static.php?page=CodeStyleEnforcer) which uses the dxCore extensions to provide real-time feedback on your codes adherence to coding standards (fully customisable).