word! it depense on the field. some layouts are nice for 3-4 lanes, some for runnig directly into the 50s if you would post a pic of a fieldlayout, we could talk about the perfect tacticYou really can't decide until you get to the field you are playing and walk it. No point deciding to have 3 at the back and then finding only 2 suitable bunkers!
General advise to bear in mind when walking your field for 5 Man is to bear in mind 1 person equals 20% of the other teams firepower. This means if you can clip someone on the break you have a distinct advantage if you all make it in. As the fields are not so long as a 7 man youhave a good chance of getting someone.Think about where they are likely to break to (ideally a lane 2 of them will go through) and stack 3 or 4 guns on it. Use what's left on the other side of the field or run them to an advanced bunker. Unless you really can't do it do your best to have all 5 guns rolling on the break!
With regards to formations decide when you see the field. Practise break drills/laning and communication for now.