If you don't actually need 64 bit (much memory or heavy on 64 bit data manipulation), don't. As the instructions are larger, the program will be larger, using more memory with no performance gain at all.
This might change in the future, as memory becomes a less important limitation, but I don't expect that to happen soon. It seems like any advance in memory technology is used up before it even hits the market, as lazy coding makes even simple programs memory hogs. For example, a file browser shouldn't need 30-50 MB per window, a web browser shouldn't use 50 MB per tab (and I've even seen Chrome use several hundred MB, in one case 1.3 GB for a single tab...).