I don't think there's any reason not to manually submit to as many search engines as desired or possible, other than the fact that it's very time consuming. Most engines have a "submit your site" link on their home page and many are (or were when I did it) free. Once the information is provided, they guarantee to spider your page within a certain amount of time, usually one to ten days if I'm remembering correctly. Also, some of the engines are 'related' (in terms of ownership or by contractual arrangement) to one another, so one submission takes care of several search engines at once. This cuts down, a little, on the time, but it is still a tremendous task. As noted above, this will only index you... it will not improve your ranking.
Just my $0.02... I'm certainly no "SEO Expert."