I don't think it is "unsafe" to set all the services to manual - if you do however you will probably be in for a rather sluggish boot up process while each required service is manually triggered. The best thing is to follow the advice of sites like edge gave you to find which services are rarely used or not required.
Here's one example i found:
http://www.joejoe.org/forum/index.php?showtopic=1203