This is hard to answer, because people differ in how they like to mix, and how they like to vape. And sometimes you won't know which method you like best until you have tried things for yourself.
But I can tell you what I do, and what I have been thinking when it comes to buying my next batch of nic:
I vape at 10-15 mg, flavour extremely low (5% is typical, except when it comes to my home-cooked tobacco extracts and Hangsen's concentrates that are recommended at 30% and I use 15%). My experience of commercial juices is rather limited, but of the 5-10 I've tried I haven't wanted to vape a whole carto of any of them at the original flavour strength.
Right now I have 36mg/ml PG nic (and zero nic PG and VG). I like having the latitude of aiming for a juice of 1/3 strength of my nic - means I have some room for adjusting PG/VG ratio.
OTOH, sometime I feel like I use a stupid amount of unflavoured liquid diluted to 14mg/ml in my daily vaping - it's just the right strength for me to combine with some 10mg/ml 5% batches of peppermint and orange I whipped up. And I use it a lot in toning down juices that I feel are too strongly flavoured.
Next on my list to get is a good 30-50 ml dropper bottle for making this to me essential unflavoured liquid, I get tired of mixing up 12ml all the time (the most my biggest bottle practical for carrying and dripping will hold).
Now, I am treating this like a hobby and mixing all kinds of weird stuff (I have cocoa powder steeping in VG at the moment), so obviously it is practical and economical for me to get a comparatively high nic and then dilute as I go along. I am still undecided on whether I will stick to 36mg/ml or make the next order something like 50mg/ml.
But what if I tired of this? What if I decided that mixing Hangsen concentrates* to unflavoured nic is all I have time for?
Then I think I might be tempted to order some 18mg liquids, and go up in either nic or flavouring a bit just for the ease of mixing (adding 15% flavouring to an 18mg/ml liquid gives a nic level of approx 16mg/ml, adding the recommended 30% gives a nic level of ~12mg/ml - so unless I was such a flavour-weenie, this would be a very easy set up for me).
Or I might figure the cost savings of buying a high nic level and cutting it myself would be worth the extra step.
So you see, Enlightenment, it is really hard to give advice on this!
For maximum flexibility always go with the higher nic.
For maximum safety and minimum work involved go with the lowest workable nic concentration
*The Hangsen tobacco flavours I have tried are pretty good - I chose according to what analog brands I liked and it seems that Hangsen's chemists have a "nose" or taste pretty similar to mine, they are not of course exactly alike, but the similarities are impressive. Ecigsoutlet.co.uk has a good range of these.
Before anyone ventures into diy, please read the info contained in the stickys above. 
