Bettas are very good at adapting to high pH. Just make sure when you get the little guy/gal to acclimate it the right way- Place it in the tank full of dechlorinated water for 15-30 minutes in a bag or cup. After that, little by little add a small bit of the tank water in your fish's cup/bag/whatever and do that for three times in intervals of 5-10 minutes. That way not only has your fish adjusted to the temperature, it has also adjusted to the possible differences of water quality.... Just make sure the bag/cup isn't too full when you first put it in, but filled enough for your fish to swim around and get air when it needs to- I've made the mistake of having too much water in the cups and it filled up on me before I was finished acclimating it >.>;
I still do this with my betta when I change his water. It's not 100% necessary, but I do it for an added precaution. You never know if you accidentally put too much or not enough dechlorinater in comparison to last time or if there is or isn't slight differences from your water source this week in comparison to last week or how much of a difference it'll make if your fish is used to the old tank water vs. the 100% clean dechlorinated water...
Edit: Also, adding chemicals that mess with the pH can harm your fish- it's best to leave it alone 'cause you never know if the pH might crash on you otherwise.... The sudden jump in pH can really shock your fish's health, even to the point of death....