The main three considerations when decided how much to boil are
1- how big is your kettle?
2- how big is your stove burner? Some people can't boil more than 3 gallons on their house stove.
3- how will you chill the wort? If you're boiling 5-6 gallons, and not topping up with water, or using an ice bath, it's really hard to quickly cool 5 gallons of boiling wort.
If you can boil 5 or 6 gallons, then by all means do it! You may have to adjust the hopping a bit (we can help you with that) due to the better hops utilization in a full boil, but it does tend to make better beer.
Broken Leg Brewery
Giving beer a leg to stand on since 2006