If the prio. will be enough after the next block don't force fees.

If the user was really after the fastest possible confirmation times
they would be manually setting a fee. In cases where the wallet builds
a transaction with a priority that is too low to qualify as free until
the next block, go ahead without a fee.  Confirmation frequently takes
multiple blocks even when a minimum fee is provided.

src/wallet.cpp

@@ -1154,7 +1154,10 @@ bool CWallet::CreateTransaction(const vector<pair<CScript, int64> >& vecSend, CW
                 BOOST_FOREACH(PAIRTYPE(const CWalletTx*, unsigned int) pcoin, setCoins)
                 {
                     int64 nCredit = pcoin.first->vout[pcoin.second].nValue;
-                    dPriority += (double)nCredit * pcoin.first->GetDepthInMainChain();
+                    //The priority after the next block (depth+1) is used instead of the current,
+                    //reflecting an assumption the user would accept a bit more delay for
+                    //a chance at a free transaction.
+                    dPriority += (double)nCredit * (pcoin.first->GetDepthInMainChain()+1);
                 }
 
                 int64 nChange = nValueIn - nValue - nFeeRet;