That really depends on the aircraft. We are required to have at least one crossfeed open and the crossfeed manifold pressurized for takeoff and landing.
You really can't pose a question like this without specifying an airplane, because the reasons and the practices are going to vary with the specific fuel system, and in many cases, the type of fuel feed operation in progress (depending on the fuel load). We may be feeding from the crossfeed manifold via the jettison/fueling manifold from a center tank, we may be feeding directly tank to engine, and it all depends on the specific airplane and the fuel load we're carrying as to which of five fuel feed configurations we're using at any given time.
We have at least one crossfeed open all the time with fuel supply and pressure to the crossfeed manifold because in the event we do need to crossfeed, it's not the time to start trying to get fuel there. Putting fuel in there from an empty manifold would take about 20 seconds and about 600 lbs...rather than end up in a situation where we need the fuel and don't have it...we keep it full and pressurized all time.
In situations in which the center tank fuel will be used, it's fed through the jettison manifold to the crossfeed manifold, to the engines. In situations where the inboard tanks have more fuel than the outboards plus their reserve tanks, then the inboards are feeding themselves and the crossfeed maifold via the jettison manifold, and the outboards are feeding from them via the crossfeed manifold, etc.
Some systems have crossflow or crossfeed open all the time, some only when a crossfeed or balance operation is in progress. You'll have to be aircraft-specific, because it really depends on many factors, starting with the aircraft system, and ending with what one wants to do with it.