So how do you know your soil is being affected to such degree that the pH is wrong for whatever you are growing.
As I understand sheet composting, its likely to use up some degree of what nitrogen is available in the soil.
That would suggest the pH is higher.
But, to what degree is it changed to any extent unless you test it regularly.
Even then, testing is not done in a general area but taken widely to prevent overfixing.

Lime given lawns ia a good general habit to get into if the lawn is regularly given high nitrogen fertilizers which can indeed change the pH when given on a longer term basis.
Its not something to be concerned about when feeding a lawn over a short period.....say...2 years, three years.
When lawns are forgotten about, and fed regularly high nitrogen fertilizers, then getting the pH fixed may be a something to think about.
Giving lime as a general fix-up, doesn't do any harm....not that it actually does any good is subject to question.
Without a proper soil test, one might go too far when giving the lawn lime just because you think it needs it.
The health of the lawn, how it stands up to adversity, how it prevents weeds getting started, how it maintains its greeness and fills in those bare spots that always shows up in spring is, I think, a better measure of whether it needs any fixing.

So how do you know your soil is being affected to such degree that the pH is wrong for whatever you are growing.
Answer: 3 soil tests done by county ext.

btw: the area i'm concerned about in the backyard is a veg. garden. i haven't sent a sample of backyard for soil testing, but after having 3 different soil tests from around the front yard (where there wasn't a lawn initially established) and seeing how poor my crops did last year i'm still assuming it's very acidic...

actually i looked up this year's soil test from where i posted it on gardenweb:
P = 9 (low)
K = 227 (medium)
pH = 4.8
buffer pH = 5.8
Ca = 1556
Mg = 252 (high)
Zn = 1.2

Calculated CEC 13
%BS 35
%K: 2
%Ca 26
%H: 65
%Mg: 7

Recommendations
---------------
N: 1 to 1.5 lbs./1000 sq. ft.
P2O5: 3 to 5 lbs./1000 sq. ft.
K2O: 1 to 2 lbs/1000 sq. ft.
Lime: 220 lbs bagged lime/ 1000 sq. ft.
Zinc: none
Mg: none

Unless you were to spread "Quicklime" which I doubt you could buy anyway it takes a fairly long time for the lime to react in the soil and change that pH. The most bestest time to spread lime is in the fall so it does have some time to start working on changing the soils pH, not in the spring when no change will take place during that growing season.
The pH measures the hydrogen ion activity in the soil and the more of that activity the lower your soils pH would be and lime has a property that reduces the hydrogen ion activity. A low soil pH does not mean you have more acid in your soil, even if those terms are a fairly easily understood (and misunderstood) way to explain what is going on. For lime to effectively change the soils pH it does need to get into the soil and, while many of us do spread it on the soils surface, tilling the lime in would be most effective. However, I have not tilled lime in for years and found that eventually the rain, and other water, will get that lime into the soil. If I do need to lime a planting bed I will spread the suggested amount on that bed and cover it with compost and mulch which seems to help get the lime into the soil faster.

From the U of F :

How do I Know if my Soil Needs Liming?

Remember that it's really not the soil that needs the liming -- it's the plants that grow in the soil that may benefit from the liming effect. Liming is needed only when your soil pH is below the recommended range for the plant species you are growing. To be sure your plants will benefit from liming, consult a reliable growing guide for the species you are cultivating. Next, take a representative soil sample and have it tested for pH. Home test kits are not very reliable. Professional tests are well worth the time and minimal cost. A reputable soil testing lab will want to know the kind of plants you will be growing in that soil before giving you a recommendation concerning liming. Be careful of advisors who recommend alteration of soil pH without knowledge of the plants to be grown.

You might find this link can answer some of your questions .

Here is a link that might be useful: Lime and Liming -- A Florida Perspective