Depends what you want to protect it from if it's
1. Moisture - Unless applied well toe guards can allow moisture in and make it difficult for the moisture to evaporate. Varnish will soon crack and again allow moisture in and not so easily out. I have never seen an unprotected bat with a rotten toe. I have seen rotten toes with the original toe guards still hanging on.
2. Cracking - Don't try and drive Yorkers and knock it in properly
3 Feathering - Apply a toe guard, apply it properly ensuring the external edges are completely sealed and remove it at the end of each season or when/if a gap appears.
That's my opinion.