It depends what you want to do with the language. For some applications, like auxiliary languages, irregularities, redundancies, etc. are best avoided. Languages for fiction should usually mimic natural languages though, so a certain amount of imperfection is desirable. My fault as a conlanger is my perfectionism. I have a hard time letting flaws into my languages.
On the subject of contractions, I include instructions in my languages for words that can be dropped in casual conversation. For example, in FairyLang, the verb "ni" (to be) is often redundant and can be dropped, unless you're being formal.
Re: dialects: A lot of conlangers create dialects of their fiction languages, although to what extent those dialects are described, I don't know. My most complete lang, Oltengo has a high and low dialect, but I don't have the rules for the low dialect written down anywhere. I just have a general idea in my head of how it differs from the high.