Consider this snippet:
After adding $1$ to $2^n + 1$, we get $2^n + 2$.
Then this:
After adding $1$ to $(2^n + 1)$, we get $(2^n + 2)$.
Is there any writing style convention that prefers one over the another. I am trying to know when we should to delimit an inline math expression with parenthesis?
The primary aim of all writing, not just mathematical, should be clarity. With that in mind, the question becomes whether the parentheses help the reader understand what's being said, or if they might add ambiguity.
There's not actually much difference between the two examples you've given: in neither case would you expect the reader to parse the sentence differently (mostly because any mathematically skilled reader will view the $+$ sign as binding more tightly than the words around it and so automatically group the expression together). I would personally prefer the first form though, as the parentheses in the second form add weight to the expressions: because the parentheses are not strictly necessary I assume that you've put them there to draw my attention to that particular grouping [insert standard disclaimer about assumptions here :)]
In fact, the only instance I can think of where the parentheses perhaps ought to be mandated is in inline fractions, such as $1/(n+1)$, even though it might be clear from the surrounding context (e.g. "...incrementing the denominator to $1/n+1$ refines the partition uniformly..."). This avoids garden-pathing the reader, however briefly.
As a last example, I took this from a paper open on my desktop at the moment, and it seems not to suffer from a lack of parentheses: $\|x_n\| = \max_{t\in[0,1]} |\frac{t^n}{n}| + \max_{t\in[0,1]} |t^{n-1}| = \frac{1}{n} > 1 $