last time it use to be starcraft 2 for example, we had a SEA version. But then they made it all regions which you can select from battlenet app which region you are playing from.

So your case, your bnet account is asia region since that is what you registered. But you are still able to select in battlenet app to play from whatever region you prefer e.g. US, europe or asia.

I play on US region because english speaking and more players (not to mention most of my online buddies from US region xd)

latency wise .....  when playing diablo 3, the australian server has a latency of 75ms and below which is much faster than 200 ish ms when playing on the US region server.

the only downside is, if your playing in AU diablo 3 server, but the 3 other people aren't, their latency will suffer alot. So only do this if the people in game is mostly going to benefit from a lower latency connecting to the AU D3 server which is closer to them e.g. asia/AU/NZ for example.

But to connect to the diablo 3 australia server, you need to manually add in the australia IP to force connect there. If your australian it will auto connect to the blizzard australian server.

My example talks about diablo 3, so it could be different for hearthstone but not sure.

But for hearthstone, although sometimes there is delay (i won't say i never experienced games where there was any distinctive lag), but overall i think you won't notice lag 99.9% of the time, and is very playable.

So in the end imho play US region.

In regards to your account being registered as SEA battlenet, i don't think you should be overly concerned about that. At most, maybe when they do beta give aways, they will only flag your account as being from asia. Last time beta keys they use to prioritize based on regions your battlenet account is registered to, but these days it seems to be global, so i doubt this is a problem these days.

hope that helped 
*update

Okay i login to my bnet account here
http://us.battle.net/en/

i did notice one thing, that the credit i pay for is in USD. So i think if you register bnet account as US, then the currency you deal in will be in USD. Might sound bad, but actually back when Diablo 3 had an auction house, what this meant was when i sold items i get paid in USD as well. So for me that was the reason i registered my bnet account as US although i'm actually in Malaysia 

So that is the only difference i can think of