I also change my alamo ressies all the time.
I check the rates on a regular basis after I book by going to the Alamo site and starting a new ressie. I'll usually try it with several different codes and coupons, because Alamo sometimes drops a rate for one code but not another. If I find a combo that has a better rate than the one I already have booked, I go to the part of the site where you can change an existing ressie and try to apply the cheaper codes.
Sometimes when you change your codes on an existing ressie, the rate you get is not as good as the rate you get quoted for a new ressie with the same codes. If the rate from changing the codes on my existing ressie is not as low as the rate I was quoted for a new ressie, then I book a new ressie with the lower rate. If I do have to actually book a new ressie instead of just changing my existing ressie, I make sure not to cancel the old ressie till after I have booked the new one.
I do not, however, keep more than one car ressie at a time. I may change a lot, but I think it's a little irresponsible to keep multiple car ressies for the same trip, because the more people who do this, the more likely the companies are to increase rates due to increased bookings, even though many of those bookings will be cancelled or even no-shows.
I'm not a terribly experienced traveller - I only fly to Orlando and I only rent cars in Orlando - but having used Alamo, Dollar, Thrifty, and National over the years, I like Alamo best, especially since they are now on-airport. The first year or so that they were on-airport, thier rates went up considerably, but since last summer they have come back down and Alamo is often the cheapest on-site company, and sometimes is cheaper than off-site places like Thrifty. The drawback to that, of course, is that Alamo is one of the busiest companies, which can mean standing in line when renting a car. But heck, for a $20 or $30 savings (sometimes more!), I'm willing to stand in line for as much as 30 minutes, even on vacation, since that's the amount of time I would save over going to an off-site rental place.
Anyhoo, change your ressie, even if it's only $5 cheaper. After all, $5 is $5 - that's a gallon of gas, 2 Cokes, or most of a Mickey Bar.