To ask your questions, depends on your SQL servers and connection attached with the servers. There are a lot of questions to keep connection alive until it is expired.

One significant problem I encountered is I got 20 locations and each location has several computers to logon my SQL server in HQ, in old day, our company using one of management program open connections without closing it, which cause a lot of gateway conflict, resulting NAT problem, and crash SQL servers... at first, we think this is due to our connection, then we think our hardward, then we think of our windows version... all stuffs changed to new now... the problem is still ON, then, what we do?! Open hundreds other gateways to handle the connections, the problem became less...

so... you don't need ODBC, due to you can connect SQL server via DSN-less.

Using SQL-PT on quering SQL, close connection when it is done.
Use remote view for updating tables, and close views when the form done.

I would think it is better to release everything... since you don't know what will be happening when it is in client sides