I would generally agree that running Sales Update during the day is not a good idea. But if you're completely confident that your data selection would cause sales update to only read orders that aren't being touched by any other UBE or application (I think UBE's or a bit more concerning because they don't lock records whereas applications do). If you think about it, during the day most companies already have pickslips running, backorder release, and maybe even invoicing, which all hit F4211.
But on the other hand, I would agree with what others have said that you should address your availability problem specifically. There are three things that we do to try to keep availability as accurate and as real-time as possible.
1. Relieve inventory at ship confirmation (P4205) rather than Sales Update. This can be done via processing options and helps to keep inventory more accurate during the day. You can even set the processing options to relieve inventory at both steps and not worry because if inventory was already relieved it won't relieve again.
2. Run repost sales orders (R42995) nightly when NO users are in the system and NO report is running that does anything with inventory. If anyone is entering orders, repost is trying to chase a moving target. Run it wide open, meaning don't confine it via data selection such as by orders, etc. That may only make things worse. You could however run it by item or groups of items.
3. Created a report to look that runs hourly to look for "odd" availabilty, like negative commitments or negative onhand.
Using the approach above, we are able to keep our inventory pretty accurate. If you have more questions about any of this, just let me know.