In general, I don't think a wider than desired rim is better than a narrower than desired rim, but it seems in my experience that tires of any particular size vary a fair bit depending on the manufacturer especially in cross section. In other words, a 90/90 x 18 tire from brand x can be distorted much differently than the same tire from brand y on the same wider (or narrower) than optimal rim. That's just an opinion based on my own experience and is totally non-scientific. Certainly a tire with a higher aspect ratio (the lower the second number is the higher the aspect ratio) will be more affected since the sidewall will be shorter. You can imagine how much different the installed cross section of a 110/80 tire will look on a 2.5 or 3.0 rim - how adversely this will impact the performance of the tire is hard to say and no doubt would depend on the manufacturer, but it is not something I would want to do. That said, I fit a tire that is appropriate to the rim, and if I am determined to fit a wider or narrower tire I change the rim width. I wouldn't worry at all about miss-matching rims aside from wanting them to look the same.
Keep in mind that your bike originally came with a 1.6 front and a 1.85 rear (from memory - which never was that great) and drove very nicely. Going up one size front and back, and fitting appropriate tires is a much more substantial change than may seem to be intuitively obvious as bikes as a rule are fairly sensitive to tire and wheel sizes. And remember, the wheel weight - which is all unsprung weight is at least as, if not more so, important as tire size and might even be a lot of the impact of fitting larger tires as bigger tires are substantially heavier. So when you go up just one size in rim width and fit appropriate tires, you are adding both wheel and tire weight both front and back and you can imagine that not being inconsequential. Plus, adding insult to injury, you will have to run inner tubes on the spoke wheels. All that extra weight is weight that your suspension has to deal with, and as likely you will be reducing the overall weight of the bike - none of it from the wheels, the unsprung weight ratio will become smaller which works against you even further. So you have a very strong incentive for making and keeping the wheel/tire package as light as possible. The smaller and lighter the bike, the more important the wheel weight is. In general, I go up one size in rim width if I am building new wheels, and fit the matching tires. Part of that is due to the tire size availability. A 90/90 x 18 tire is a mighty small tire these days!