What matters here
Respond promptly, thank the host, and decline clearly without inventing elaborate reasons. Where the relationship is close, a warmer personal note may be appropriate.
Rules are useful starting points. Judgement comes from combining consideration, context, culture, consequence and the actual relationship in front of you.
Questions to ask yourself
- Who is affected by the choice I make?
- Is this a social convention, an organisational policy, a cultural expectation or formal protocol?
- What would reduce awkwardness without creating a larger problem?
- Is there a host, organiser, policy or specialist who should be treated as the authority?