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There may be an advantage in fitting a 6mm dowel into a 1/4" (6.35mm) hole.
What I generally dislike about dowels for joinery is that they are all-or-none. If not perfectly aligned, then they do not fit together. Dominos and biscuits have a little adjustment built in.
So, fitting a 6mm dowel into 6.35mm hole offers 0.35mm leeway each side. Not much, but it can be just enough for better alignment. I doubt that strength would be impacted.
Regards from Perth
Derek
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I've purchase imperial dowels through Amazon Australia....
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Not so easy with hardened guides.