A lawn seed blend very common is a mix of Kentucky Blue Grass, Perennial Rye, and a Fescue. Around here this is called a "West Michigan Blend" but I have seen the same mix sold in other areas as "Southern Ohio", West Indiana", Northern Illinois", "Western Pennsylvania", Western New York" blend.
I have not found it necessary to overseed an established lawn if the soil is properly maintained since the grass growing there will fill in if given half a chance, often faster then seeding will.

KBG is a sod forming grass that usually is so dense you don't need to reseed in the fall. It is the fescue grasses that need to be reseeded. Fescue is a "necessary" component only when you have shade. KBG grows in full sun but fescue is more shade tolerant. If you have full sun then the KBG should be excellent for you.

One problem with blended seed is the different seeds sprout at different times. Rye sprouts in a couple days, fescue sprouts in 10 days or so, but KBG takes several weeks of nearly continual moisture before it will sprout. What usually happens is the homeowner sees the early grass sprouting and tends to back off on watering. Then the KBG never comes in to take over the yard.

Spring is a poor time to reseed simply because all the weed seeds in the yard will take advantage of your frequent watering of the grass seed. Thus you will have a weedy lawn the first season. Another reason it is a poor time is the new grass roots are not strong enough to withstand the summer heat. It is likely you will have 70% crabgrass by August. Whereas if you seed in the late summer to fall, the crabgrass is dying out and the new grass has all winter to develop root structure.

The only reason to seed an established KBG lawn in the fall is to have green grass over the winter. It does go dormant whereas fescue and rye do not necessarily. But then you will have to mow all year, so that is a consideration.

cool, thanks for the info.

i had a good amt. of crabgrass in my yard this year and i've done nothing to stop it really, so i'd say i'd have tons of i seeded then, my front lawn isn't that bad off to have to deal w/ more crabgrass.
but my back lawn is, so i might just have to grin and bear it with my backyard this year.

also, i have about an acre of slight hillside that has poor soil like my front and back lawns did, but im not really interested in paying out to get it in ideal shape, or have to water it a lot, it's mostly covered with thin grass and weeds, is there something kinda maintenence free i could throw out there to kinda fill in the gaps?
one day i plan to make it look nicer but right now, im trying to focus on the front and back yard

Perennial Rye is added to many seed blends because it is a fast germinating grass seed and that can aid in stabilizing the soil while the other seeds do germinate and grow. Fescues are added because it is not a good idea to have a lawn with only one grass species in it because that makes the lawn too susceptible to lawn diseases and pest.

dchall, i'd actually like to mow it, i really like the way it looks mowed, and well.....i like mowing haha. so i'd like to stick with something more basic, maybe red fescue would work but it work on my area considering terrible soil and full sun??

im planning on just putting a common sold bag of KGB down on my front lawn, would you suggest putting the same on the hillside? or what about these contractors mixes i've been seeing at walmart? what are they about???

West, most of the labels on those "contractors" mixes that I have seen state they have a lot of Annual Rye Grass in the mix, a very fast germinating grass that is considered a "weed" by most lawn growers.