There is always a good time to snipe, actually - but you have to be creative and find a time and a PLACE that's right for the game situation. You're not playing deathmatch, you're playing ONS, so if you want to snipe, help your team.
Early Game: Get that link gun out, get your first nodes built, then get over to where you know the main enemy advance is. Even if they're just advancing to build more of their locked nodes, taking a few headshots will slow down their build, and will get people looking for you instead of linking nodes. One guy stalling 5 or 6? Great thing to do.
Midgame: Stay in those primary areas, the lines between where the enemy is spawning and where the battle is. Sniper cover on the main battle really isn't as effective, because they EXPECT to be taking fire then. They also somewhat expect to take fire right when they spawn, from campers. They don't usually expect to get shot in the head halfway in-between.
Endgame: Winning endgames should be a sniper's paradise. Spawn camp them in the base, or better yet, snipe them between the base and the first nodes, where they'll expect it less. You should have a Hellbender to replace your rifle, too, if it's a large enough map with plenty of spare vehicles.
Losing endgames aren't the place for a sniper. Put your sniper rifle away and get another body in that big fight for the first node...unless you happen to see sniper fire coming down. If THEY have a sniper, pick him off, then keep moving. If you start to take that first node, then it's a judgement call - many skilled snipers are better off taking up a sniping spot and slowing the enemy's counter-advance than trying to fend them off at the node. The fewer guys that make it to the node, the better your buddies do, after all.
Snipers are NOT just out for their own scores. Some are, but I believe any ONS team should have at least one or two sniping specialists on it, and they shouldn't be discouraged from using their skills.
Snipers out there...remember my words, and when you see the whiskers...headshot the guy beside me, k?
