When should I use "很" before an adjective?
Yes.It is common for "很" to be used before both monosyllabic and disyllabic adj. But when these types of adj./adv. are used, the front "是"are always omitted. For example，我很优秀（I am excelent）, 我的国家很强大（My country is powerful). This mechanism covers most  of the context. But you may use 我是很优秀，or 我的国家是很强大。It is not grammatically incorrect,but you will hold a tone of sarcasm (or at least a serious tone),so there's smaller chance to use this format . 

When the speaker uses 我是很好,我是很优秀 or 我的国家是很强大。He/She wants to express the determined tone to the recipient,in which he/she consolidates his/her authority,or against others' questioning for the most of the cases. In other words,he/she is boasting or even be conceited about the advantages of his/her or his/her contry.