What wine should be used for deglazing, e.g. a tomato sauce?
Will certain aspects of the flavor (e.g. fruity, dry) be noticeable in the finished dish or is it just for the 'general' wine taste?
Can I use a wine that has been opened a few days ago or is it better to use a new bottle?
Unless the recipe calls for a fortified or sweet wine (e.g. madeira, sherry) use a dry or very dry wine, usually white, but not always. You can easily ruin a dish using a sweeter wine, as I once did with risotto.
Whether or not the flavor of the wine comes through depends entirely on the wine used and the quantity. If the recipe calls for a cup of wine, then you will probably notice it in the final dish. If used just to deglaze a pan, then you probably won't notice it at all.
Old wine is fine as long as it's palatable and/or hasn't soured. Beer works great as well.
I'm not sure what you mean by deglazing a tomato sauce. Are you making a tomato sauce from the deglazing? It's worth looking at what deglazing is and why you do it.
Normally you would deglaze a pan for two reasons. First, to recapture all the flavour from whatever you were cooking in the pan that has become burnt or caramelised or all the meat juices that have escaped. Second, to produce a sauce from that flavour.
What wine you choose to deglaze with (and it doesn't have to be wine, it can be stock, marinade, even water) is wholly dependant on what you've cooked in the pan and what sauce you intend to make with it. The lighter more delicately flavoured meats such as chicken, turkey, pork, rabbit etc. tend to work better with white wine. Darker, more flavoured red meats such as lamb, beef, venison, boar etc tend to work better with red wine. This is a general guide, there's always exceptions.
A couple of points to remember when deglazing with wine –
I certainly wouldn't open a new bottle of wine to deglaze with. I deliberately keep old bottles of wine with a few inches left in the bottom for precisely this purpose. I've never noticed any difference at all between using fresh and old wine for deglazing.