We show that for the special case of linear PCFGs (which include HMMs) with non-degenerate priors the posterior puts zero mass on non-tight PCFGs, so tightness is not an issue with Bayesian estimation of such grammars.
because all of the commonly used priors (such as the Dirichlet or the logistic normal) assign non-zero probability across the whole probability simplex, in general the posterior may assign non-zero probability to nontight PCFGs.