how worthy are shortened links from a SEO point of view? Should I avoid shortening links whenever possible?
As far as internal linking is concerned I think it's good to mix them up. Have some links with just keyphrases in them, others with complete sentences derived from the keyphrases, others not related to your keyword target at all. Don't be afraid to play around with graphic links with well constructed alt attributes. Throw those in too. If you use the title attribute in your anchor tags then do it consistently across your site, or don't do it at all. Short links, long links, targeted links, random links, graphic links, well worded title attributes. Mix and mash to keep the search engine interested. Organize your internal linking to focus on your important web pages but do it naturally and with the intent to impress an Internet visitor. Don't force feed links to the spider, let it develop a hunger for your content. Let it find the new links in a reasonable volume.
Remember that you don't need many incoming links to satisfying Google off-site factors. The best links are always your own and the ones that get generated naturally from satisfied Internet visitors.
Now if you are building yourself some incoming links, then the same principles would apply, avoid repetition and create innovative anchor text. Don't do like everybody else does. Make it all stand out, your webpages are likely amongst a big crowd pumped-up mediocrity so try to make it all look more intelligent.