We did blown in cellulose last year. It made a huge difference in both winter heating AND summer cooling.

We did it as a condominium association and received $3200 dollars in credits from People's Gas in Chicago.

Home depot will loan the machine for free if you buy 20 bags of insulation - if you are feeling spunky.

Does blown in insulation effect that?

You should definitely check for exposed wires or open boxes (you should do that anyway).

You can work around the stuff later and/or replace where needed. It just gets a bit messy.

All our residential electrical here in Chicago is in conduit so pulling wires later is easier. Most of the country uses Romex (non-metallic cable) for residential.

We did some work at my mother-in-law's house down-state where Romex is used. The electrician we had on-site suggested mounting any new junction boxes in the attic on the roof rafters rather than burying the boxes in the cellulose. This makes finding/access easier post insulation. I suppose you can flag/mark any fixtures or boxes for future reference.