I wouldn't recommend it. You'll likely want to move away from SendGrid for transactional email in the future (I wrote a whole article about it here: https://news.ycombinator.com/item?id=15329869). If you are going to get locked-in to a provider, just save yourself the trouble and put your transactional stuff on Postmark.

There are other services for abstracting away your templates and transactional logic. For example SendWithUs.com. Also check out Litmus.com. Or wait for my product!