Do you guys use a custom DNS while connected to a VPN?

I've always just left the VPN's default DNS settings alone but recently started wondering if there's any actual benefit to using something like Quad9 or Cloudflare DNS instead.

Does changing the DNS while using a VPN actually improve anything or does the VPN's own DNS basically make it pointless?

Also is there any downside to mixing the two? don't really want to mess with my setup if its not doing anything useful.