You're asking the wrong question. Let's assume we have some keywords like these, from google keyword tool (GKT):
- digital camera
- digital camera reviews
- best digital camera
- compact digital camera
- dslr camera
You could have them like that in the anchor text or you could combine them as:
- best compact digital camera reviews
- best dslr digital camera reviews
- best digital camera reviews
And you can have the both ways and it will help. Some links with the exact keyword, others more long tail. Regardless if you target the long tail or not. From my experience, if you have a lot of links with long keywords, you will automatically rank for the root keyword as well and vice verse. I think combining is best.
For example I never optimized or targeted the root keyword in my market (e.g. digital camera) because I never thought I would outrank the big players. However, though my site is only PR3 and has only 3000-4000 backlinks, I am ranking #5 for the root keyword and many top tier keywords I never tried to optimize for. That's because my anchor text varies (covers many keywords) and because I have a lot of content on this topic and my site is a useful resource for a visitor. Basically google sees me as an authority in this market. The sites on positions 1-4 have between 50,000-1,500,000 backlinks and at least several hundred or thousands of links to the page that ranks for that keyword.
Combine the keywords but make sure you always have in the keyphrase the main keyword you try to rank for. Also, make sure that any strong links you get (from powerful link exchange partners for example) have the exact keyword you are targeting as anchor text.
These days I don't even bother trying to rank for a articular keyword. I try to rank for all of them, including the tier 1 keywords (root keywords). If possible have double listing in the SERPs and fill the entire 1st page with several of my sites. So I have for example site1.com ranking on #1 & #2, site2.com ranking on #3, site3.com ranking on #5 & #6. After I get several sites to rank like this, I then just interlink them using in-text links and that will give me a big boost, making me rank for other longer tail keywords and be hard tooutrank by a new competitor.
The downside is you need some serious infrastructure to do this. I am working on my backlink aquisition and management system for the past 10 months and is only 60-70% complete. Aside from that system I have other related systems planned that will take at least a year to complete, but are worth it. In the end I will have several hundred websites on my own hosting/domains (different C-classes) plus 1000+ WPMU, web2.0 and parasite hosted blogs. That's a lot of backlinking power. The concept is not new, Eli from bluehatseo.com probably made it public first, but the details are what make it work or crash. In the end I will have a system that will make Scrapebox, Senuke, xgenseo and xrumer look like toys for retarded kids. It's just so f***ing exhausting to keep working on it for so long

but as I said it is worth it in the end.