I think it looks a little choppy. I had the same situation with white appliances and finally just bit the bullet and replaced them all at once. It was tough to swallow as all were still in fine condition. So, it's a tough call whether to just get a new oven in white or live with them mixed for a few years. I guess it depends on how much the white is bugging you. In my house, it really began to bug me. I've sure enjoyed my switch to stainless.

Would definitely look better if they all match, but it can be hard (expensive) to replace all your appliances at once. If you can only do it one by one I wouldnt worry about it.

The real problem will be that any white you get won’t match the old ones maybe just start by getting the stianless ones you need and replace the others when you can. Do not get into the new slate, black stainless as these will be the same problem as the white down the road no matching found