I think betta's are fairly tolerant to Ph levels, Mine is 7.4 straight from the tap (thats getting on the high side, but they do great in it.)... alot of folks have levels in the 6 range with no problems,
Betta's come from areas of southeast Asia which naturally have pretty soft water with a pH range roughly around 5.8 up to about 7.0 (which is neutral water). However, bettas are hardy little fish and will survive in higher pH's rather well, all the way up to somewhere around the 8.0 or I've even heard 8.5 range.
Now, a little water chemistry for the curious about how your tap water's pH can change (some of you may want to skip this). Here where I live water companies pull the water directly from the ground from the aquifer. Fresh from the ground the water here has a pH of about 7.3 and a TDS (Total Dissolved Solids) of about 500. This is way too much TDS, do you like drinking sand in your water? This water is then run through RO (Reverse Osmosis) at around 120 PSI all the way down to 1 micron filters and by the time it comes out the TDS has dropped down to about 50 and the pH is about 8.0. Various chemicals which I will spare you all from listing as they are long, obnoxious and really quite boring are then run through the water to help make sure any bacteria and/or other toxic chemicals are removed from the water, this drops the pH back down to around 7.8. These chemicals are then removed (through aeration and other various means) and chlorine (or chloramine, depending upon which local district we're talking about) is added. The USA government requires that all public drinking water have a minimum of .2 ppm (parts per million) of chlorine in it, but not more than .4 ppm. Chlorine raises the pH of the water, although not all that terribly much. When the water is finally deemed safe for human consumption and shipped out into the pipes, around here the pH is about 8.3 on average.
This may be a bit high for betta's, true, but I've never had any of my bettas have issue with it. Although by the time my tap water goes in the tank it's no longer got an 8.3 pH anyhow. Putting a water conditioner (aka a dechlorinator) in your tank's water neutralizes the chlorine, and drops the pH a bit. Also, I take my water from the tap, leave it in a bucket with the water conditioner and let it sit for generally a day, or sometimes only a few hours if I need to do a water change now. This aeration (and any further aeration it will get whilst being in your tank, especially if you have a filter to move the water) will also help any other chemicals that are in the water to help dissipate back out, and this also will generally lower the pH a little bit.
*cough* Now I'm sure that was way too long and overly detailed for this question, I'll shut up now.
I'll attest to the hardiness & adaptive qualities of the betta. My water tests at a steady 8.0 and both my bettas do just fine. I enjoyed the narrative on the process of water treatment, Nataku. Very informative.
My water is about 7.6 I believe. I was once told that it was so important to keep the pH at 7 and I even bought tablets that supposedly "made" the water a pH of 7. One day I was curious about how long it lasted and two days after I dropped the recommended amount it was back up to 7.6. So I just stopped worrying about it, and all my fish have done great . . . even my neon tetras.
pH stability is much more important in most things (at least freshwater) than whatever the actual number is. I wouldn't be concerned about needing to adjust pH unless your pH is reading above an 8.8 .... in which point in time I'd also be wondering where your water is coming from and whether you should be drinking it. xP
Drastic swings in pH are like drastic swings in temperature for many fish - too much, too fast will will shock and possibly kill them. Having a certain pH tends to be much more important in saltwater tanks (where you will be introduced to the wonderful migraine-inducing fun of pH-drips and buffers, but we shan't get into that here) and in certain more picky species when breeding - which betta are, thankfully, not.
Secret weapon? Oh no, they've found me out! Retreat, retreat! xDDD Not really. But glad to know some of you found it useful information.