Occasionally you may notice your bridge leaning forward or backwards. This happens in response to string pressure and tuning. When a bridge is properly fit and carved it will stand so the back looks to be square to the joint between the top and ribs. The rib miter joints are usually parallel to the back of the bridge, but this depends on proper construction. When everything is done correctly the force of the strings passes through the bridge without pulling either forward or backward. If a bridge is not fit or carved properly it will warp more readily.
You should occasionally pull the bridge back so it is straight again. This must be done carefully. If the strings are cutting in to the top too much it will create a lot more friction, and make this hard to do. With both hands on the sides of the bridge and the instrument held securely in your lap you can pull the top of the bridge back towards the tailpiece. It's not a bad idea to put a pad under the tailpiece just in case the bridge were to slip. This pad would prevent the fine tuner from hitting the top and causing a crack. If you're having to use too much pressure it may be best to ask your luthier to walk you through this to make sure you're doing it safely. Once you understand the technique, though, it's not a big deal.
You need to make sure that the feet are still making full contact on the top. If you have to choose between a bridge that is leaning or a bridge making full contact on the top, get as close to straight as you can with the feet making full contact. This prevents the feet from digging in to the top and potentially causing damage.
If the bridge has a warp that prevents you from pulling the bridge straight you may need to use moisture and heat to get it truly straight. Generally don't recommend that anyone straighten their own bridge. This is mostly due to concerns about proper bridge placement, and the possibility of a soundpost falling down. That being said, it's not complicated. Assuming that the bridge is properly made you can dampen the concave side of the bridge and place it under a warm incandescent lightbulb with the convex side close to the bulb. If the bulb is too hot it is possible to have a small crack form in the bridge in which case it should be replaced. With that in mind you should leave an inch or so between the bulb and the bridge.
If that method does not bring the bridge back straight in 30-60 minutes you may need to try something else. You can soak the bridge briefly, then clamp it to a flat board, and place it under the heat as in the previous method. Cello and bass bridges may soak for 5 - 10 minutes. Bass bridges with adjusters should not soak where the adjusters are. If it is warped in that area it can be tricky to deal with. Soaking the threaded area of the wood may make it very hard to turn. It can also cause a crack around a metal insert if the wood is soaked.
Occasionally the second method still leaves a warped bridge in which case you should see a luthier. They may try something else or recommend a new bridge.
And keep in mind, a small change in bridge position of 1/4mm can make a significant tonal difference. You really want to make sure the bridge goes back where it belongs. I sometimes use post it notes to mark bridge location. Just place them right up against the sides and the back of the feet, and you'll know right where it goes. To reduce the risk of a soundpost falling you can lay the instrument on the treble side rib, so the post will stay in place. If a post falls, though, it is a sign it is overdue for replacement. Not something you want to deal with in an emergency, but not something to ignore either.