Generally speaking it is good to stick to one engine manufacturer as far as possible, especially if ordering new.
When leasing used aircraft it is down to your judgement. If the aircraft is abundant then get common engines but the cost of operating two or even three manufacturers isnt usually too much if you end up with 2/3 aircraft of each engine type. Basically its nowhere near as bad as adding a fleet group so not something to loose sleep over (the faster expansion will generally speaking far outweigh the costs of different engines).
Hope this helps