It varies.
When I'm running an old-school D&D game (AD&D 1e, Rules Cyclopedia, most OSR, etc), I don't worry about level differences much at all. Those games are designed to quickly catch lower level players up, and the level differences don't matter nearly as much as they do with D&D 3.x and 4. A lot of the games I run don't have levels, games like RuneQuest and Burning Wheel where your skills go up as you use them (a simplification, but good enough for the purposes of this discussion). In those cases, there are no levels to worry about.
One thing I've notice in the games where I'm a player (and this is completely subjective, YMMV, and all that). In my group, there are a couple people who will run 5e. In both cases, they use party XP and/or party level. Currently, one of those GMs is running a Ravenloft campaign. The GM was giving me a lift home after this last Saturday's game when he mentioned there's some post-raise dead effect he'd like to try out, but no one in this group ever bothers with trying to get slain group members raised.
I hadn't thought about it, but it's true. Out of the last few 5e campaigns, and several 4e campaigns prior to that, only once has someone bothered to get a slain party member raised. The problem is, the players of the slain characters never care that their character has died. One GM used to be apologetic when a PC was killed, but he'd get back an "eh", a shrug, or other reactions along those lines. A bit of time with a character sheet/builder/spreadsheet, and he's back in action, no fuss no muss.
O death, where is thy sting?