Most guitars strings shrink & stretch over their lifetime. They usually stabilize after installation but bends & general play can cause the strings to stretch or the tuning knobs to give a little. With that said get used to tuning regularly, religiously, by ear or by tool.
Tuning References
Standard
E – A – D – G – B – E
Thickest < – > Thinnest
Drop D
D – A – D – G – B – E
Thickest < – > Thinnest
Drop C
C – G – C – F – A – D
Thickest < – > Thinnest
Open D
D – A – D – G – C – E
Thickest < – > Thinnest