sendgrid uses password based authentication (config details for that below). It won't care about your IP, or the hostnames you use (myhostname, myorigin, etc).
The hostname your server presents to sendgrid in the HELO (or EHLO) greeting is likely to appear in mail headers. Some recipients' spam software may check it, so use something that does exist in public DNS, and not associated with a dodgy domain, but few systems will notice this stuff. Good to do it well, but not critical.
I have a configuration where I send mail through sendgrid for multiple domains, so postfix needs to know which sendgrid account to use for each sender domain. (and if it's not a known sender domain it sends via a local relay, without using sendgrid). This is not really what you asked for, but my guess is that the problem you think is about domain names and your shifting IP is actually about authentication with sendgrid.
relayhost = smtp.example.com
smtp_sender_dependent_authentication = yes
sender_dependent_relayhost_maps = hash:/etc/postfix/sender_relay
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymous