If you measure your nutes well and change it like you said, you can do without the EC meter, but you will need a pH meter, or a good method of testing pH.
When my tapwater is pH 6.5 (sometimes wanders to 7.1), the pH buffers in the Canna nutes I use set the pH to 5.8 when I mix for 1400ppm (400ml A&B in 125L rez), requiring no further adjustment.
However, soon after we've had some heavy rain in the catchments, the water co treats a bit more strongly for pathogens that get washed into their reservoirs. pH jumps to 7.1, occasionally to 8.0. The pH buffers in the nutes don't cope as well and I need to correct down.
I'm with pottie, a pH meter is an essential. You can get away without a nute meter if you know someone using your nutes who has verified the mix quantities. However, as your tank of nutes is consumed by the plants or evaporated, nute strength will bump up if the plants don't eat the nutes as fast as the water is used. Without a nute meter, I'd top up frequently. Nute concentration can't do anything but drop if you top up frequently.