The thing to understand is 95% of what you read on the IPO forum is people promoting (or defending) their financial interest. "Tight register" is just another line from the promotion playbook to make punters feel justified in chasing a stock when it lists.
The problem with the tight register argument is, even on the occasions where it's true, it often doesn't translate to a strong performance. Just because a broker has, say, $6m of demand for a $4m raise at 20c doesn't mean there'll be an extra $2m of demand in the aftermarket, at higher prices. The $6m of demand was there at 20c - not 30c. Not even 25c. Most smart investors and other brokers bidding into a book know roughly where fair value is in the ST. They also know easy money when they see it. This is why an IPO book can see big demand at the IPO price but much less at each incremental price rise. It's also why you often see gaps-up in highly promoted IPOs and then a fall shortly after. What's happening is a small amount of money is still willing to chase at higher prices but is met with supply from those set at 20c (or seed levels). Price starts to fall, those who've just bought stop out (or end up holding the bag) and seed and IPO holders reduce further, as they can see momentum is waning.
As to how you evaluate a tight register. Most of the time nobody knows. Brokers (good ones) I talk to are often surprised how well or poorly an IPO does versus expectations and promotions. Occasionally, you just know. PAC's AGH listing just had a feel about it and even brokers were getting $5k allocs and you had to fight hard for anything. Even that didn't guarantee anything though. PAC's MRG also had crazy demand and scaling and opened soft.
I actually disagree that a tight register is the best way to run an IPO. When I hear about how a lead manager will banish any sellers to the pits of Hell, never to get a deal again, it's more a red flag than anything. Running a craypot IPO might look good for the first few weeks but it's not sustainable. A lead manager with fire power and decent insto relations (and a quality offering) should be able to secure a strong aftermarket that is well bid and can handle supply from those exiting. So unlike some, I want to see early supply and corresponding demand.
Lastly, register tightness isn't super important in the long run but don't believe for a minute that people aren't selling to you just because they claim to be long term holders. Many are selling on the way up, to derisk their seed holdings that have often been held for 12 months or more. The only truly sticky stock imo is escrowed stock. Best to trade with that assumption and if a stock ends up tighter than expected then it's a bonus.
