Usually, yeah, but not necessarily (e.g., for those with dedicated IP addresses).

Even then, it usually will be since most customers will just add the (customer-specific) "include:nnn.wl.sendgrid.net", or even just "include:sendgrid.net". But, for whatever reason, I've come across several that add an "a:" or "ip4:" for their dedicated IPs instead (I assume there's a good reason for that, but I don't know what it is).

Probably so that they can also send some mail from their own servers, in addition to sendgrid's.