How to determine a protein's cellular location based on its sequence?
You could use Blast2go to find the predicted relevant gene ontologies of your sequences. If you look for the cellular compartment sub ontology you should find the location of such protein. I have never used this tool, but here is the paper describing how it works.

I wouldn't restrict to the first predicted match of blastp see if some more genes with a good fit are also located at the same sites. But otherwise your workflow seems right to me.

But to prove this is in X or Y you, or your team or collaborators, should always check with histochemistry if it really allocates on the predicted place. All the softwares predictions must be validated with some biological experiments to prove them.