If you’re comfortable with the top-down tile map and visually it meets your goals then there is no reason NOT to use that for your game. However, isometric style or first-person 3d appear to be the more common standards today.
If this is just for hobby or to prototype an idea then you can probably stick with top-down. Likewise, if your target audiance is for mini devices ( PDAs, cells ) or web applets you can probably keep w/ a top-down for even a “real” game as long as you implement well. However, if you are trying to develop something that will be targeted for desktop play and expect to have sale value I would think the market is pretty much past top-down RPGs.
You may be able to seperate much of the game’s logic from the graphical representation and migrate to an iso view later, for example. Especially if you try to keep presentation seperate right from the beginning. it would probably be easier to stick with iso view from the beginning if that’s your target, but if you just want to get a working game to check out character interraction, world management, combat and quest systems, network issues, etc then it may be worth building top-down first if that’s what you know now.