I don’t believe it’s explicitly stated as a system-wide rule, but rounding down is mentioned in factoring Reflexes, Mortal Wound, Health, Resources, and Superficial Wounds, plus determining Aptitude and starting Rank for multi-root Skills.
Rounding up is mentioned in spacing Light, Midi, Severe, and Traumatic Wounds, but that’s so that the numbers don’t crowd at the bottom of the scale.
IMS says that you round UP for Incidental and DOWN for Superb, but I always think of it Moderate being the baseline and adding/subtracting half-round-down of Power to get the I & S.
Only other rounding rules I’m aware of are ones that change the baseline, such as the Tough trait allowing you to round up Mortal Wound.
That said, Diminishing Returns is sort of rough already. If a player is dead-set on repeating a lifepath enough to be affected by Diminishing Returns in a normal 3-5 LP game, I’d let them float the half-points and round down at the end instead of rounding down on each pass as a compromise. So getting DR on 5 + 5 ranks I’d treat as 5 total (2.5 + 2.5 = 5) instead of 4 (2.5=> 2 + 2.5=> 2 = 4)