In general, I'd say writing, especially on the back, is more acceptable.
Some folks on here actually pay a premium for 'ink' (stamps) on the back of certain older cards as that generally indicates some provenance.
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Working Sets:
Baseball-
T206 SLers - Virginia League (-2)
1952 Topps - low numbers (-1)
1954 Bowman (-5)
1964 Topps Giants auto'd (-2)
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