Are instance variables set to nil by default in Objective-C?

I'm sorting out some memory issues with my iPhone app and I've just been thinking about some basics. If I setup an ivar and never end up using it in the lifespan of my object, when I call dealloc on it, will that cause a problem? E.g.

@interface testClass {
    id myobject;
}
@property (nonatomic, retain) id myobject;
@end

@implementation testClass
@synthesize myobject;
- (id)init {
    ...
    // Do I have to set myobject to nil here?
    // So if myobject isn't used the dealloc call to nil
    // will be okay? Or can you release the variable without
    // having set every object to nil that you may may not use 
    ...
}

...

// Somewhere in the code, myobject may be set to
// an instance of an object via self.myobject = [AnObject grabAnObject]
// but the object may be left alone

...

- (void)dealloc {
    [myobject release];
    [super dealloc];
}
@end