To a degree the need for an AI will depend on the dosage, 100 mg a week of Test will not require AI because the level of test will be fairly close to a natural amount.
A very high dosage of test, say for example 1g, will generally require an AI because the residual testosterone will aromatise to become estrogen which will be in excess and lead to undesirable side effects like moodiness, bloating, gyno etc.
Although generally the effects of excess testosterone are desirable (I.e. enhanced muscularity) the effects of excess estrogen are often not, you will need the correct amount of estrogen to feel well and your job is to balance your AI to achieve that.
I’ve found a good starting point is 12.5mg aromasin once or twice a week with 600mg of test E, but this needs to be adjusted for you and ideally blood work helps.