It depends. A lot of people don't hold with adjusting it unecessarily. Do you know the PH of your water?
A betta should be fine from 6.5 anywhere up to 8PH as long as it is stable. If your water is around 7.5 I wouldnt buffer, personally. However, if it is below this you are getting into the territory where a PH crash could cause a big issue. Fortunately, carbonates can be added naturally, but still comes with all the risks of faffing with water chemistry. Adding something to buffer your water will not mean you dont have to monitor it.
Before adding anything, I would recommend PH testing your tank once a day (at around the same time each day) and before and after the next water change. If your PH doesn't fluctuate, I would do a PH test weekly before any W/C, and leave it at monitoring. I have seen cases where PH fluctuates rapidly, even within 24hrs, in water under 3ºdKH, at which point the lack of stability would make me want to think about additives. If your PH stabalises at a suitable level, regular water changes and 'ageing' your water should be enough on their own.
Your PH level, and its consistency, is critical here.