[Krsqk]

Natural chokepoints are the obvious choice--the problem is in quadrants such as grid which have few if any systems with fewer than 3 warp points. IMO, border defense in these situations becomes an economic issue--if you can outproduce the other guy, you can support fleets to defend larger borders. Larger borders, in turn, mean greater economic strength, which means larger fleets, which mean larger borders, etc... It feeds off itself, usually with diminishing returns as your borders grow geometrically.