Generally speaking, no, importing with IOM does not need to be relegated to off hours! The tables are not locked during importing unless you are Validating Import, or you leave an interactive box waiting for input.
That said, there are a few things to think about:
* How robust is your database server, and how busy is it? If you already find RE to be slow when there are a lot of users in it, then adding on imports at those times could cause you to see a performance degradation in RE. Just as running a large report or query in RE would cause one under the same conditions.
* The RE user running the IOM imports will not be able to use RE for anything else while the import is running. Can you afford that during business hours?
You could create a user just for importing, and then have the user launch a second session of RE on the same machine and log in as themselves while the import processed. This doesn't always work well, and the workstation will need to be fairly robust to support two active RE sessions (particularly if the non-IOM session is going to be reporting or querying).
NOTE: You can run into RE database locking if an import is left with an interactive screen/pop-up box awaiting input from the user. This is something that would be a very real possibility if the import user was logged into more than one RE session.
* Validating an import can cause the RE database to experience locking, as Sharifa indicated above. Validate Mode is really intended only to test an import to be sure you aren't going to get tons of exceptions due to a configuration mistake in the profile or an issue with the incoming data. The bulk of your imports should be run without this option checked. Always create the applicable output queries (Output--->Queries section of the profile configuration), and import all gifts to Batch (Gifts section of the IOM profile), for the greatest safety net.
For more information on Validate Mode, and the potential locking, please see this post: http://www.omaticsoftware.com/Forums/tabid/108/aft/466/Default.aspx#1239
* You may wish to consider the IOM Scheduler add-on module. This would allow you to run imports in the background during the day, or unattended overnight. Any rows that could import without any user interaction (don't need to click on any pop-up boxes) would process, and all rows that needed input from a live person would be output to the exceptions file and could be imported later. If most of your rows are going to require interactions then Scheduler might not be much of a time saver for you, but if the majority of them will import without interaction, it's great!
More information on The Scheduler: http://www.omaticsoftware.com/Solutions/TheRaisersEdge/ImportOmatic/Scheduler.aspx
Hope this helps!
We've just started using IOM for our bulk imports (so using it for approx half a day every day). We are finding that it slows down users in RE when importing or vallidating - has anyone else experienced this?