As i know update to 7.5.4 quite stable. I've updated both my servers without any problems. Update just replace some files in Plesk and doesn't affect any data. Not sure if i'm really lucky men but updates doesn't bother me.
Update plesk 7.5.3 to 7.5.4 is safe: I've done it on 2 server and all goes well.
I've only stopped all the Plesk service on the machine and in 15 minutes I've got the server up again.