Certainly! No flags needed: each year when a new Gibbons Part 1 comes out, I check my collections against it. I also place tags in my stockbooks against the better items, even if they haven't yet crossed the threshold value.
This has a double value. Not only do I sweep up all the new entries for the spreadsheet (a bit laborious then going back and adding in the prior catalogue year data, but as I said, can be very instructive), but just occasionally, I find things I've overlooked previously. For example, this year I was running through my Bhopal collection, looking through these common 1884 ¼ Anna laid paper perforated stamps (SG 51)

when I turned up this

Before, I'd just pushed it aside as a nasty case of trimmed perfs. This time, I looked at it properly, realised it couldn't have trimmed perfs (it's a longish story, but trust me!), and was able to catalogue it as the rather scarce
imperf version, SG 50. The perforated version, SG 51, is catalogued at £1; real world value, maybe $0.25. The imperf version, SG 50, is catalogued at £225; real world value is hard to say, because they come up so rarely.
Now, the question of actual values. First, you have to remember that I'm talking only of the Indian States - but I know them quite well. Scott is useless on the Indian States, so it can be ignored; Gibbons is the only general authority. The Indian States market isn't quite as big as, say, the United States market, and there are far fewer items offered for sale. That makes it much harder to establish a 'true market' value from
ebay, or from the auctions, for that matter.
Nevertheless, you can probably generalise that for many Indian States, sold by reputable sellers (in this context, regrettably, that tends to mean Anglo-Celtic sellers in the UK), full current Gibbons catalogue values tend to be about the norm. Unpopular States sell at a discount, but unusual items (covers, multiples etc) even of the unpopular States may actually sell at a multiple of Gibbons.
Gibbons inevitably lags the market, and the market being so relatively thin, Gibbons has trouble valuing everything. However, taken over a five- or ten year-period, the catalogue values are a pretty accurate reflection of the market trends for a given item.