I did not come across such situation any time but without powering down some of VMs, the host on which you are going to migrate powered on VMs will be super loaded and VMs may not perform well. there also can be impact on vMotion operation, it will not be that smooth as it can be. As you are going to put host into MM, it will not be stopped by VC itself due to memory constraint (admission control is also not enabled) & if memory is over committed, alarm will be generated from memory perspective.
If it is not production environment, then you can go ahead put host into MM and post your observation. (but you will have evacuate that host first manually) (I am sure in production, admin usually do not allow cluster to corss 90%)
Also you can enable DRS on that cluster and put host into MM, DRS will automatically start evacuating that host. It is better if you could analyse if you can power of any VMs from either host.