insecticidal spray/ fly strike spray???

Is the small animal insecticidal spray good to use??
Iv also been looking at the fly strike spray..would this be useful?
One of my buns has gone a bit downhill lately so I don't know wether he's keeping himself as groomed as the other 3, I was thinking about getting some spray and giving them all a few squirts during this time of the year.
Good idea or not?
If so how often should I be using it??

i use the fly strike stuff,the expensive one as i think it last the longist,you spray the every 12wks and personally i think its a good idea,specially if there outside a lot,i sprayed my two on sunday with the weather getting warmer and ive seen a few of the big flies out,so much to there annoyance they got sprayed