You are worried you will cut something out that you didn't want to cut out and losing it forever...here are some of my favorite tips to offer-
If you are worried that an unreplaceable photo will be ruined, don't crop it. Use it as is. If you want you can even mount it with corners or in a mounting sleeve so that someday you can make copies of it (then you can crop those!) Most photo stores can do copies of prints without negatives.
Start out using photos you're not very worried about.....recent shots that you've gotten doubles printed of or something.
When cropping consider the composition of the photo....don't throw the baby out with the bath water! Remember that sometimes backgrounds hold things we want to see years down the road. If getting an oval means cutting off those cute baby toes or some other such thing either don't do it or stop the oval at that spot and cut out the toes by hand so they're still in the cropped photo.
If you are a good photographer, you may not find it desireable to crop your photos. I have been working at improving my photography and I do less and less cutting as I get better. Cropping is still wonderful to salvage those pics where the kids moved the wrong way or someone wandered into the view of the lens (or I find something embarrassing in the background after the fact) but its not necessary for all photos. The best photographs are generally better left untouched, just mounted in a way that accentuates them.
Throw out the really bad ones! They won't get any better even when there's less of them!
I agree with BevS97 about most shapes....unless your layout and your photos are particularly suited to them they don't generally turn out very well. I'm also pretty averse to cutting photos with fancy edges...the paper I mat the photos with looks great decoratively cut, but doing it to the edge of the photo generally detracts from the photo. On occasion a page full of balloon, heart or star shaped photos can be cute, but its easily overdone. I try not to use more than one "shape" (circle, oval or other) on a page.
Keep in mind that the whole point is to make your photos more enjoyable to look at and to help your photos tell their stories.
BTW...I'm a huge fan of the CM CCS and all that goes with it...used selectively.
Oh, and as Norm Abrams would say...."Measure twice, cut once"
Have fun! That's also the point of it!