You don't necessarily need to go all the way to the end of the cogset before shifting the chainrings.

If you see a hill approaching, drop down to the smaller chainring, then shift to harder (smaller) cogs in the back, until your cadence or pedal effort is similar to what you were doing before the chainring shift. This way, you have more gear choices without having to shift the chainrings while on the hill.

With more experience, you'll know which chainring to use in different road conditions - flat, shallow uphill, steep uphill, rolling. You'll still switch chainrings as needed, but you'll more often be in the middle of the cogs, making shifts up or down a cog easy.

You want to avoid extreme crosschaining, like small chainring to small cog, but doing it occasionally won't hurt anything.