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If there are no links/references to your site, the engines won't take your site very seriously... In fact, they may not even index it.
Depending on what phrases you're competing for, you may or may not need a lot of inbound links to rank well. Ultimately, for the long run, you will need both.
SEO should be part of your focus from the time you start to design the site. Ok, you'll do brushing up, refining, etc, over time... but getting an idea of your keywords and building them into your copy is done before the page even goes up, designing your page structure, formatting your headings to use H# tags..... why would you put up a site and then come back to do these things later? Cut out the double work and you'll double your time.
A properly SEO'd page is actually quite resilient and doesn't need a lot of ongoing attention, whereas linking should definitely be an ongoing strategy - both checking your existing links and gaining new ones.
It's not difficult to get a link or two, which is all it takes for the SEs to find you.. and then you can build the links from there. But the links won't have the value you expect if your pages aren't SEO'd for the appropriate terms.
The other thing, of course, is that a well-SEO'd site will invite links when prospective partners are searching for "widgets add url" and your site comes up tops.
Long and short of it:
1 SEO in design and page building stages of site.
2 Linking strategy on launch of site and ongoing.
Remember, too, that links are significant to Google, but of secondary importance to the other SEs.
An oversimplification would be to say -
Keyword relevance factor X PageRank = ranking
If relevance for keywords is zero and PR is 10, ranking will be zero.
Vice versa, if keyword relevance is 10 and PR is zero, ranking will be zero.
Both need to go hand in hand for overall performance.