HASELL checks are a precursor to aerobatics, spinning, spiral dives etc when the aircraft is likely to be in a position where avoidance may be compromised. In the case of a steep turn you are training the student to fly a co-ordinated level turn with the main objective of being able to avoid someone or something at any level. The aircraft is under control and introducing inappropriate checks may delay the necessary action. Students will subconsciously recall the first demo that you give them. Lookout is essential as always.
I would not teach a student to do steep turns at 500 feet initially, because the teaching exercise includes spiral dive entry and recovery along with stalling in the turn; items frequently omitted by the FI. The objective is to teach the student how to do it correctly before moving on to the upset situation where appropriate checks should be conducted.
A student ready for a licence skill test should be able to demonstrate a steep turn at 500 ft following a good lookout, and used to be conducted as such on the UK PPL skill test as part of the low level section.
In the case of a stall (base turn or otherwise) you are training the student to recognise and recover from a situation that they should not find themselves in wheras, the steep turn is a manoeuvre they should be ably to fly with certainty and precision.