Are you maintaining domain names associated with nameservers for other businesses? If you’re doing then keeping a separate account at domain registrar is important. Mixing it with your domain investing account is somewhat not advisable.
I used to maintain other people’s domains with my domain investing account – but, I faced problem, when they asked me to give domain access to their technical team for some development purpose. If such thing happens then you have two choices – transfer the domain to their preferred registrar or push the name with in the same registrar website.
The domain I was maintaining was associated with a website and email. The domain registrar was different and hosting was at different place. In order to not disturb the emails – I pushed the domain to another account with in the same registry by keeping the nameservers intact. The process took 30 minutes.
During this period – the emails and website was down. But, it resumed immediately once the domain moved completed. If a domain is transferred away from the present registrar then it usually takes a day for nameservers to be updated. Godaddy puts a 10 day lock on domains that are moved with in their network to prevent any fraud. And, as usual any transfer will have a 60 day lock.
The best practice is to open a separate account for any of your customer preferably with their credentials. And, then giving away the login credentials to them or maintaining it safely by enabling the 2fa on that.