Not really. It's just a question of codon frequency. Organismic codon bias ensures that some codons are actually used quite rarely in the genome already. Specif

Not really. It's just a question of codon frequency. Organismic codon bias ensures that some codons are actually used quite rarely in the genome already. Specifically, the frequency of TAG stop codons is ~0.03% in E. coli, and the frequency of TGA is 0.1%. By comparison, the frequency of CTA leucine codons is ~0.3%. So it's only 10-fold more effort than the easiest codon, and ~3X more effort than the next easiest. And it may really not be necessary to recode all of them.