This is kinda like the 'do I need to prime?' or 'do I need to clear?' questions.
The answers to all is 'no', you don't NEED to. But, if you do prime/clear/use micro-set sol, you can get much better results than without.
I've haven't used the micro set/sol stuff yet, but I use Gunze's equilent. Mr Mark Setter and Mr Mark Softer.
These products make the decal look painted on, there isn't that much of a 'gap' between the decal and the paint. Most important is that these products help the decal stick to abnormal shapes. Search a little on how to use these products.
As for the longevity, these solvents wouldn't make much difference.. clearing the decals after they have been applied would. Gunze has recently release an 'anti-UV' clear. That might help even more. NOTE: I haven't used this product yet so I do NOT know how it reacts to decals.
As for where these can be purchased,
www.hiroboy.com carries them. There are plenty others which I don't know of.
HTH,
Steve