I wouldn't use tap water - except for a few areas in the country - it's wildly inaccurate, especially right now with most of the country experiencing drought.
I've had a lot of heartbreak at the hands of the municipality with water during a drought - they're basically grabbing water where ever they can get it (which to be honest, their primary concern is -having- water, not having good water for planted tanks).
There's also other contaminates in the tap, such as fluoride, which have profound effects on Nature Aquarium, almost always negative. I don't even suggest drinking tap water.
While splitting your RO to tap ratio at 50/50 or 75/25 is certainly an improvement over just tap, if you have the means, always stick to RODI. Basically, we need to treat planted tanks like Reef tanks - except instead of salt, we reconstitute some mineral bases.
Following a pattern of (once on once a week water changes) doing 50% one week + equilibrium then the next week 50% straight tap, the results are incredible.
Growth patterning of plants is also effected by the use of RODI - such as Rotala Colorata, where in harder water or non RODI water, the stems grow much larger in diameter, but with the use of RODI the growth is extremely fine and tiny, the same is seen in HC and Riccia.
Nature Aquarium is complex enough without needing to add on the headaches of inconsistent tap water qualities with no control over what goes in or out. Short of living in Seattle, where there's a 67% chance of rain everyday and a fairly-well preserved environment, I wouldn't use tap ever. Not even as a small consideration.
However, if you absolutely cannot invest in an RODI system of some kind (or have a LFS to pick up RODI water from, or are unwilling to get distilled water from the local grocer), then my advice would be to plan to build that into your budget as soon as you can. Take whatever measures are necessary - it will pay you dividends, I promise.
Tap water is a last resort, not a go-to commodity. With many of my clients at this point I say that RODI is a necessity, not a luxury. I won't even use tap water in Houston (which averages 8.6-9.0 out of the tap) to do an emergency water change. Better to leave the tank without a water change.
Additionally, the use of Penac W in the substrate system actually does quite a lot to help reconstitute and fortify RODI water. By way of neutralizing acidity in aqua soil it helps to mineralize water, so when using Penac W / P in your substrate, you actually reduce the amount of re-mineralization you need to do by quite a bit. The product was actually originally used in Niigata to help fortify the water that they were using (though it's primary purpose isn't as a re-mineralizer) to enhance the environment for plants.
Sorry, I rambled.