Yes if you leave the cap open on the liquid test kit it will be invalid. I run a ph probe( test at 4.0 and 8.0) with my apex on my s/w system but it is far more necessary for the tank in comparrison with the pond. Ph swings in small amounts will competely destroy a s/w reef and is not a need with constant monitoring in a pond application.
When i was a kid, we did test kits too that used used the test strips but in todays standards it is not even close. I 100 percent agree with our friend that has done the different testing via liquid test vs test strip. I will get the same results as she would get on my s/w system.
By all means, if you feel more comfortable using those strips....... Then use em. I think in today standards that test stips are more basic, but when you get involved with make adjustments to you homemade ecosystem and you want to keep a constant variable in check, i would use the more accurate drip method of testing.
My question to you is, if you have mass ph swings, would you feel positive using the test strips to dose properly? I would not.