Do you need one? Of course not. Would it make a difference? Probably. In a shorter distance event like that difference would be small. What's your bike like? What's your current position like? Can you dial that in further? An aero helmet can make a pretty reasonable difference over a longer distance event (especially for the price compared to other upgrades). The aero gains are personal though and vary a lot from person to person. Without actually testing it yourself, it's very hard to say for sure.

Are your Olympic distance triathlons draft legal? If so, the helmet will make far less of a difference overall than sticking with a fast pack.