5 feet 10 inches is 70 inches, and 70 inches × 2.54 cm = 177.8 cm. Most forms want 178 cm (rounding to the nearest whole centimeter). If it asks for a decimal, 177.8 works.
A quick check: 1 foot is 30.48 cm, so 5 ft is 152.4 cm, and 10 in adds 25.4 cm, which gives the same 177.8 cm.
The inch-to-centimeter conversion is exact by international agreement (since 1959), so the only rounding is the one you choose. Your height could differ by a few millimeters depending on how you were measured (shoes on or off, posture), but for a form, 178 is the number to enter.
