Alertness and Stealth

I have been thinking about using a skill system in my next campaign; I was trying to come up with a couple of skills, when I got this idea. The general rule in most OSR D&D-esque games is that parties are surprised on a roll of 1-2/d6, which is essentially a d6 roll against a Target Number (TN) of 3; these numbers are hard coded, but I wanted to play with them a little, which resulted in the following:

Alertness could be a skill which decreases the chances of surprise (by increasing the die size step by step); it could be rolled against 3 by default or higher in case of stealthy creatures (5 for giant spiders lurking on the ceiling, for instance).

Stealth, on the other hand, could be a passive skill, increasing the TN the opponents have to roll against to avoid being surprised; the TN is 3 by default, 4 for skilled, 5 for expert, and 6 for master skill level.

Although noting these numbers for individual monsters seems like extra work, it is actually already included in the rules (counting the chances from 1 and up, though). Any possible downside of this I do not see? How would you try to implement such a rule?

Thief Skill Rolls

Another quick post. Thief skills and the method their success is determined have been discussed many times on either blogs or forums. This particular post is aimed to improve upon Jack's idea of using the "hear noise" column for everything instead of the complicated and quite arbitrary percentile tables, about which Brendan of Necropraxis (earlier Untimately) also posted about.