Is having a consistently lower than normal diastolic blood pressure a good or bad thing (i.e between 40-50 mmhg) if the systolic pressure is the normal range (i.e in the 90s mmhg)?

Prob normal for you. There are some red flags associated with a low diastolic blood pressure in older people (may mean arteries are stiffer, or taking too much of an anti--hypertensive drug(s), but in a young male such as yourself, it is likely normal. Especially since your systolic blood pressure, while normal, is on the low side, too. Unless you have symptoms such as dizziness/light-headedness, I would not worry:)