Most of you guys know I'm anal about water chemistry, so it should come as no surprise that I adjust the mash pH every time I brew. There was a period of time a few years ago when I checked the pH of my water, adjusted, checked the mash to confirm Brunwater, checked the first runnings, checked the kettle and sometimes adjusted, checked the post boil pH and then checked pH prior to packaging. I'm a little less anal now. Some days I use my meter to confirm. Most of the time I trust Brunwater to get me in the right range based on my well water pH and the grainbill. I've found the variance of my well water pH to be less than .3 throughout the year and that's not really enough to concern me.
I'd suggest when you first start taking pH readings that you check your strike water from the filled MT. I've found that will differ slightly from water straight from the tap. Probably due to residual left in my MT(kettle). Doesn't affect it a lot but sometimes .2-.3. YMMV
Because my water is a little more on the 'hard' side I can brew some dark beers without acidifying the mash but that's because of my water pH. Your water may be softer, in which case "only adjusting light beers" is not a good MO.
I also agree with @voodoo_donut that you'll get a more pronounced flavor with a slightly lower kettle pH and have always mashed my lagers at 5.4 but lowered kettle pH to 5.2. Having said that, it also depends on the yeast that you're fermenting with. Some yeasts produce more acid than others so take that into consideration as well.