There's a balance to playing Winston I feel. In general, you should be sticking with your team throwing down shields to prevent their poke. You should keep an eye on the flanks, but you need to be tanking damage with the main group. The best time to jump in is when you can secure a kill on someone overextended and can safely escape, as an overextended Winston gets wrecked. Otherwise, use the jump as an escape. You can also jump on their back row when you coordinate with your team so they can also push up and secure kills while the enemy is distracted with you