Sometimes, this question is already answered for you due to there being only one sampling point on the unit. However, you need to consider if it is a good sampling point and thus if it will provide accurate data. For example, oil taken from the bottom of a sump will not provide an accurate picture of what is happening in the machine. Similarly, sampling from an inactive location on the unit may give positive results, but in reality your machine could be on the verge of failing.
Consider installing a sample point in a better location, budget-allowing of course. Also consider safety, especially when dealing with high-pressure hydraulic systems