It depends on how you are using them in the show. If you have a truss and run mixed lights across it that are working together, it will look really odd. However if the lights are used for different purposes from different locations, you can get away with a lot. For example, if your 250's are overhead, and your 575's are on the floor, then it won't look bad. If your beam angles are tight on the 250's and wide on the 575's, the intensity will look closer. Remember, lights always look brighter if they're aimed at you, so if some of your locations are aimed at the audience, and some are aimed at the stage (like for a band or dance), the 250's will look brighter to the audience than the 575's hitting the stage. Having a mix is often better than having only one type. There are always going to be positions that eat light, and this gives you the option of cranking things up for those positions.
EDIT:
Wow! Looks like a lot of posts came in at the same time, so some of the info is redundant, but it all looks like we're thinking the same thing!