This question came up in our last game.
In 5e, after a long rest you get to recover half your hit dice. My assumption would be is that if you are multiclass you get to pick the hit dice you want to recover.
In theory you get to pick the hit dice you want to recover even if you are not multiclass - it just doesn't matter.
Any reason to think this is not the case?
Related to What order do multiclass characters spend Hit Dice at the end of a short rest?