In a traditional RPG, I wouldn't lock the player in a place with only a boss fight they can't get away from. Chances are some players skipped a bunch of encounters and got under levelled for a boss, and it's not worth taking the risk of locking them from going back to grind for EXP. But as always, it depends on the game that's being made, and there are always exceptions to think of. In my last game, each new map is a point of no return, though bosses are designed in that they can be beaten at the minimum possible level players will be at, even if they skipped nearly all encounters. In another game I did, I have one boss fight where players are locked from returning to grind for EXP, coming directly after the first boss fight (as well as a full party revive + heal). Basically, if the player was able to beat the first boss (which does grant the option to grind for EXP before it), they should impossibly become stuck on the one right after it, since they're approximately set at the same difficulty level.