I am wondering if I should wait to hear back on submissions before I query agents?
You should generally query agents before you submit to publishers.
Once you have submitted your manuscript to a publisher, an agent wouldn't also be able to submit to that publisher - you have essentially used up your one chance with that publishing company.
How many publishers have you submitted to? If more than a few, depending on your genre, you may have really put yourself in a position where no agent will take on the book unless you get an offer from one of those publishers.
So, if you have submitted to more than a few publishers - and you can add more detail if you want more specific answers (ie, book genre, how many publishers) - but if more than a few, likely you will have to wait to query agents until you have something new that hasn't been shopped around or until you get an offer.
If you get an offer of publication - then you can tell the pblisher you want to find an agent to negotiate the contract and then query agents with a query that says in the subject "PENDING OFFER FROM XYZ PUBLISHER" and if it's a big enough publisher, some agents might be interested. But you would have to query after offer and before accepting the offer.
But absent an offer, if you've submitted it to more have a few publishers (again, depending on genre and how many are left unsubmitted to), write something new to query.
~suki