Well, setting the audit for a table is much more appropriate than turning monitoring on if the main purpose is to record the last DML time for the particular table. The purpose of monitoring is to help optimizer get more acurate information in the time between two subsequent analyze processes and hence brings much more overhead compared to auditing.

If you want to know the last DML time for the table, turning auditing on for that table is the most efficient and "natural" way to go. And besides, monitoring is available only in 8i, while auditing is available for more than a decade now.