Survey-grade GPS (using CORS and WAAS correction) using RTK will give you accuracy to ~10 cm within a few seconds. You can get that down to several millimetres if you can tolerate post-processing several hours worth of static data.
Rule about lat/long precision: If somebody says the words "datum" or "projection" to you, and you say "huh?", then you only need four digits after the decimal point in your lat/long coordinates. On the other hand, if you're designing an outdoor parking lot and you don't want puddles, please use survey-grade coordinates.
In the US, 60 cm accuracy is often possible with nothing more than WAAS corrections. 10 or even 1 cm accuracy is often possible in real time with access to an RTK network.