Is a very important aspect that is often looked over and if it was given attention, people would be able to exploit a lot more power from backlinks. Search engines like to see links coming from a diversity of IPs and that will send them the message that what you have is interesting and good content and it will be beneficial for the SERP.
As you might have realized commenting thousand of time on forums to get your signature backlink is a waste of time. Those 1000 backlinks will not give you a huge boost and having 10 or 1000 won’t make a real difference. If you want solid backlinks you need to get many from different IP sources. Those huge quantity of backlinks from same IP will never give you a huge SERP impacts. In fact 2 good signature from 2 different IP address of same value, is worth more for SERP, then 1000 backlinks from only one of these 2 site.
Receiving backlinks from the same IP can be considered spam by Search engines and they can devaluate you. Site-wide Links are not bad and will not affect you, but if 99% of your backlinks come from the same IP it will look unnatural and as you suspect the value of the backlinks will be really diminished.