Fitting them isn't really a matter of choice, however, there are two different types, and for each type there are good quality and poor quality ones.I am prepared for some leakage from the isolator valves, but I am hoping that it will be just a dribble at most (why would people fit them if they leak more than that?)
There are also two ways in which the valves can leak. They can leak from the gland or they can fail to shut - the latter will cause you a problem, depending on the extent to which it passes water.
In the best scenario, you have good quality gate valves that work. And if they do leak past the gland, you can tighten and/or pack it.
In the worst scenario, you have spherical valves that have seized, and that seize when you try to turn them and p*ss water from the glands.
Anyone who prefers certainty of getting the job done takes new valves to a job, fits them after draining the system, and walks away knowing that there cannot be any leak. Since you live in the house, you may prefer the routine of peeking into the airing cupboard every day to check for leaks. That choice is your prerogative.
If you like the sense of adventure provided by not knowing until you remove the pump whether or not your valves are working, then go ahead with the amateurish plan of buying pump pliers (which are inappropriate for removing valves anyway), and snatching the pump in/out while holding a tray/bowl/bucket an impossible position in a dark cupboard, and trying to stop water from running over any nearby electrics and motorised valves.So hopefully I can whip out the old pump and bang in the new one with only a small amount of mess ... at least that's the theory, I shall have several big buckets ready anyway .