The correct way to lay decking boards for most decking is smooth-side-up, with the ridges there to prevent rot and mould.

Does the grooves on decking go up or down?

Any decking board manufacturer or professional will tell you: “The proper way to install a decking board is ridge side down.” The ridges are designed to allow for airflow, to stop moisture sitting in the ridge, and allow for a stronger structure.

Does it matter which way you lay decking boards?

A board that is correctly installed right side up (or bark side down) will form a crown and allow water to run off the edges. If the board is installed wrong side up (or bark side up) it will curve up on the edges when dry and water can gather and sit in the cup. That is a recipe for wood rot and problems.