Is it true that if you put a 2nd class stamp on a postcard, it gets treated the same as if it had a 1st class?

I wondered this because I'm on holiday and I'm sending postcards to friends and family back home (still in the UK) and whats the point in spending extra money when I could just use a 2nd class one? My gran told me this ages ago but i don't know whether to believe her?!
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