Well, actually the print title is always centered. If you are using several poster columns, there is an option for printing the title on each page or across all columns. With an even number of columns in the latter case, the title will be split over the two middle columns. I suggest to use "File" -> "Print Preview" to test the title's "Show on each page" option.
There have been a couple of reports of user experiencing problems with their printers not always respecting yEd's page settings. Please make sure that yEd's page settings match your printer's native page settings. yEd's page settings can be configured in the aforementioned print preview.