Level 1: web
level 2: spiders (if not against hard counter)
level 3: spiders (if you went for spiders level 2)
level 4-25: see what the situation requires of you
Start with webs level 1 100% of the time. You usually want to get spiders level 2 (unless you can't play against counterpicks with AOE). If you're against something like tinker/juggernaut, you can bait them to use their AOE spells and then go on them with spiders. Make hotkeys for all spiders, spiderlings, spiderites, broodmother and broodmother+spiders. If you're going for the spiders build, don't level incap bite to more than level 1 before you have q and w maxed. (maybe not always neccesary to have webs lvl 4 before incap bite level 2).
When you're all under the tower and attacking a hero, I believe the tower prioritizes the closest target hitting the hero.
Don't rely on spiderlings to slow heroes. Incap bite has a more powerful slow.
Make sure you have 1 or 2 sentry wards at the start of the game (you or your support can buy them). Use quelling blade/ tangoes to get rid of enemy sentries.
If you're having problems with spiders blocking you, you need to improve your micro. Make sure to usually have some space between brood and her spiders.