Link acquisition planning, that is devising the method by which a site is to gather natural links is IMO a major part of an SEO strategy, and as content is what garners links, the form of its provision will influence the direction of the site and so needs early formulation.
The usual problem with link acquisition is by the time it's considered the site is already finished so limiting the options mainly to those methodologies that appear increasingly ineffectual, and unfortunately few have the choice taken by many SEOs of dumping traditional link development in favour of social media marketing.
Strikes me that while being somewhat cart-before-the-horseish, we may need to consider link acquisition much earlier in development, identify the potential sources for natural links and supply/tailor the content to best appeal to them.
Yes you rshould start aquiring links as soon as the site is live. I have found that frequently updating your site with unique content can do more than any link can do for you. Over time you probably want to have a monthly ongoing link strategy and a content addition strategy as well.