That question is totally situational therefore impossible to answer but I say spamming defence towers is always a good thing as long as you are expanding and using them to defend expansion.
e.g: every time you take a metal extractor build an llt next to it
The problem with spreading your defence too thin is that a grouped enemy army will just waltz through it. If you have a clusterfuck of towers then it will do some real damage if an enemy army walks into it.
I personally like to spam towers everywhere though, so I say go for it! Using a mixture of llt and missile turret is nice too.