If you have to put in petrol every week then is it best to fill up to full tank for two weeks?

I usually just fill up the tank to half full and it lasts a week. So do you think its best to just fill up to full tank which should be enough for 2 weeks? Or will that weigh the car down too much so its not worth doing?
Update: I weigh 97 lbs(UK) so what is your point?
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