I always, always spend a significant amount of point scouting the 4th to 7th rounds. Of course, I scout the higher prospects, but once I identify a target or two for my need positions, I start scouting everywhere on lower projected kids, need positions or not. You can never have too much depth in the NFL, imo.
I make sure to scout their key attributes and maybe speed if it's a position like WR or DB where usage can matter a lot based on speed. (I do LB too mostly because I MUST have solidly fast LBs for how I use them).
Usually, this doesn't cost many points. Coverage abilities are just 100 (MCV and ZCV), PRC is 50 I believe. You know have a strong idea how well he can cover folks. Run blocking and catching and THA's are similar. I stay with the grades on these kids since they are flyers. If they don't pan out quite as good as I though or are a bit more of a project, that's fine. It's worth the risk.
As far as determining need positions, I look at my team's production, the scheme I want to run, and of course the personnel I have now.
Mostly, I go by production and depth of talent.
For example, I identify QB on my Raiders team as a higher need. Pryor isn't terrible, but I could use an upgrade to help the offense. Because of this, I'll scout the top round kids pretty highly. If Pryor got hurt...I don't even want to think about it. Josh Johnson is worse. So, I lack depth and need an upgrade at starter. That makes QB a high priority.
I'll scout the higher projected prospects relatively deeply. I'll open up SAC, MAC, DAC on high and low round prospects. On guys I like, I'll investigate further with THP, AWR, and SPD (SPD is the mobility checker - is he closer to Marino or Manning, a 'true' mobile QB like RG III or Vick, or a pocket guy who can move well if need be like Aaron Rodgers - prefer at least the "move and gain yards running if needed).
Now let's say I had someone like Philip Rivers - I might would make QB a medium priority. I would likely bypass the 1st and maybe 2nd round prospects. I'd want to use those on different need areas. So, I would focus from mid-round on down.
If I had someone like Aaron Rodgers or Andrew Luck, QB would be a depth-only priority. That's where I'd focus on 5th and lower and look for kids to take a flyer on when it's the 5th round and it's like "what do I want to do with this pick?" I can take one of those "if and maybe" prospects.