i prefer to interchange MF gear i am clearing and surviving at a level i like; not before. Here is why:
1) Clearing speed and having FUN trumps everything. If you arent having fun you arent going to enjoy playing. Grinding slowly through trash isnt going to keep you enjoying the game. Killing shit fast is fun.
2) paragon levels + 5 stack bonus its not shabby MF in its own right. Clearing speed more than makes up for gimped MF gear. You watch some of the videos and youll see what im talking about. Its absurd how fast some these characters clear and I assure you they find more stuff than you do, and make more gold than you do. Killing speed is a huge factor when MFing and collecting gold, as well added bonus of lvling paragon faster too.
in short DPS + survivability first and MF last. Dont think about substituting MF gear until you clear and survive at a level you find desriable and fun.
I agree with jungle_woman. But if you insist on getting MF gear, my only advice would be to avoid the perfect MF gear (usually 20%, but 18% on rings and offhands, and 40% on amulets), since you pay more for that last few MF % than it's worth. Instead, look for gear with good stats and any amount of MF. Your MF will be lower but you'll kill faster and be more survivable.