wait until taken king is installed before you shard any legendaries. Those armor pieces decide what your starting level will be and if you delete them your character will be level 20 instead of level 34 on release. also those legendaries are going to be the weapons and armor you use when you first start playing TTK, you actually have to play to earn the better new stuff and you need things to play with in the beginning. also there will be enough vault space to keep your old gear if you really want. also it will still be useful in crucible and older content.

I'll play it safe and waif until after ttk drops before I start sharding.

Well, there's a couple of things you can do... if you keep them, they'll still be useful in year 1 activities and in crucible (they will scale). They may even be able to be used to infuse your new TTK weapons. If you shard them, the materials can get you more glimmer and motes of light from the speaker, which will be a useful currency (more so than now). The weapon parts will be used to up rep and upgrade weapons. So... it's kind of up to you. Both have benefits and downfalls...