Toilets, Sinks, Showers, Tubs and Disposals - Toilet flange question

I noticed that there are differences in opinion of whether toilet flange should be flushed with the fininished surface or should sit on top of the tiles? I am a little confused. Any suggestions? Should I finish the surface (leaving the flange hole?) before installing the flange? Will flange must be screwed to the floor thru tiles and subfloor?

I am replacing a 4 inch lead pipe with PVC. A plumbing supply store said 3 inch opeing is adequate but I would think 4 inch is better than 3 inch pipe (which means installing a reducer "donut." Any opinion? And, how should I install 4" PVC into the cast iron pipe? Same Rubber "Donut?"

One more question, how well does the "offset" flange work? The original lead was not in straigt line. I either have to use the offset flange or use a couple of 1/8th "bend(?)" to offset. Your help is appreciated.

Kevin

Flush to 1/4" above the finished floor, sitting on the tile is OK. 4" (general standard)would be my choice, but depends on what you are connecting to.

As to offset, the fewer bends the better, but it has to end up where you can install the toilet. So do what is necessary.