I've only just started to drive manual 5 months ago and rev matching is one of the first things i learnt. Knowing the gear ratios really help. Being in gear during a turn (with enough torque/rev) is MUST, I learnt this the scary way. I once did a 90deg turn from a 70kph street and did not bother to down shift and turned the car in 4th gear. My car understeered and I almost smacked onto the side gutter.
From then on, I forced myself to be in the right gear when taking corners. I usually brake till ~40kph drop it to 2nd, rev match to 3000rpm then enter the corner while feathering the throttle.
Now I need to take some corners more efficiently and learn to rev match into 1st gear. I haven't been brave enough to try that yet.
So is it safe to drop it to 1st gear as long as my car is moving slow enough and enter a corner in 1st gear? I only rev match btw, I don't double clutch. I find it hard to move the car to 1st unless it's travelling really slow.