it is not true at all. webman mod runs a ftp server at all times. when you enter multiman it will start another ftp server of its own. having two ftp servers running will cause problems using ftp, but i do not think that it would cause games to crash.
multiman clears some things from memory that other homebrews use. i used to reboot after using multiman (mainly to rip iso or move files) to avoid disrupting webman. i think some changes were made to webman and hen to address that issue.
BTW you should use the unofficial hen multiman fixed by joonie. it is the only version that is properly signed. (i use this version on cfw also because i do not want to accidently use the wrong one.)