If you are on foot, the general rules are these:
1) If you are outnumbered by enemies (1 of you and 2 of them) you can attack them and kill until it is even strength. Once you've killed one and have even numbers, if there are hogs you must stop fighting and take a hog. If there's only one hog, then of course you can keep fighting over it.
2) If there is a hog, you must use it. You can't walk, take teles and ignore a hog. You shouldn't be fighting unless you are being attacked, or are outnumbered (see #1)
3) You must check around the base for hogs, you can't just assume there aren't any and take off walking or attack enemies on foot. If you respawn on one side of the base, you shouldn't be nading on the assumption there might not be hogs on the other side.
4) If there are no hogs, you can nade a lone hog that drives past (but see #2, you must have checked first - you can't nade if you're not yet sure if there are hogs or not)
5) If there are no hogs you can start walking to your nav
6) If you are walking to your nav (legally, so not ignoring hogs or going out of your way) you can attack other walkers or nade any hogs that go by.
7) You can't spam nades. Let one explode before you throw the next one. You can't just throw nades hoping a hog comes around the corner and flips. You have to have a target - something you can see.
#3 and #4 are strictly enforced in our scrims, but we are pretty lenient in public servers and don't usually warn unless people make a habit of slaying, walking the map, or nading lone hogs when there were other hogs available.
So basically, near the base you must check for hogs and really try to find one and get away. You shouldn't be unecessarily killing enemies or walking the map. But if there really isn't a hog, and you've checked, and you have to walk to the next nav, you can kill other walkers as you walk (although this is good strategy, as JANE explained, you'll find a lot of people don't kill other walkers, and even think it's unsporting to do so, but it's definitely allowed.