If crowds are REALLY low you probably don't need FP since the stand by lines would only be about 10-20 minute waits. However, that is rare. When it is VERY crowded I think FP is a must and can help you do many more attractions.
Strategies to maximize FP use:
1) Get a FP upon first entering the park.
2) Get a second FP as soon as you are able and before using the first FP. Your FP ticket will tell you when you can get a new one. It may be 5 mins after your return window begins, 2 hours after getting the original FP, or another time. You get the second FP before using the first so the time on the second FP begins to run right away.
3) One person in the group can get FPs for the entire group. Just have that person collect everyone's admission ticket and take them to the FP machines. This has two main advantages. First, it lets others in the group do something while the "leader" gets FPs. They can take a potty break, get in line at a restaurant, wait in line at a non-FP ride, shop, or just rest a few moments. Second, getting multiple FPs sometimes requires criss-crossing the park. It's a lot quicker and less tiring when only one person has to trek across the park and back instead of the whole family.
4) While waiting for your return time go on a non-FP ride, get some food, go to the bathroom, shop, or take a quick break. (Some people like to wait in the stand by line of the FP attraction so they can ride twice in a row).
5) Don't fall into "the FP trap" as I like to call it. Some people see a return time that is several hours away and a stand by line wait time of only 45 minutes. They figure it is better to wait the 45 minutes in the standby line since they will get on the ride sooner. This defeats the purpose of FP since the idea is to do other things/go on other rides while waiting for the return window to open. Waiting in line 45 minutes at SM means you've wasted 45 minutes you could have used to do something else.
I'm sure others have tips and pointers they can give you.