HI,THIS IS WHISKY,MY QUESTION IS WOULD IT BE BEST TO TAKE OFF THE LATH&PLASTER WALLS COVERING BEFORE DOING A NEW BATHROOM UPSTAIRS AS OF RIGHT NOW THERE IS NO BATHROOM THERE AND IT WILL IT BE BETTER TO REMOVE WALLS AND FLOOR BOARDS TO INSTALL ALL OF THE WIRING&PLUMBING AS I SAW ON THE MIKE HOMLMES SHOW THAT THIS THE BETTER WAY TO DO IT,THANK,S

First off forget what you have seen on the home shows. The costs and time envolved is much greater then what you saw on the show. They open up the rooms to give the show a shock effect. Otherwise it would be boring to watch while they only opened up a small hole needed to do the job.
With that said,
To answer your question yes and no. Yes if the walls are in bad shape and need to be removed, No if they do not.

The plaster walls are heavy, thick and will be fairly difficult to remove. How much do you need to remove to get the plumbing rough and electrical rough in? That will determine the extent of how much wall and floor needs to be removed. If you only have to install an outlet or two or a light. Do it from the attic and fish the wires down to the locations needed. Why cut all the way around the wall for the wire.
You will need to cut into the wall for plumbing, but again stack vents can be lowered down the wall to the opening cut for access.
You can however open up the walls as its much nicer to work with a blank slate and as a homeowner doing it yourself may be the best way to go. But do you want to spend all the time and money envolved in doing so?

As far as the floor goes. I would remove the floor down to the sub-floor as you will need to cut for the toilet and bath drains. As well as access the stack that your going to tie into for drainage. Also proper floor prep will assure that the new tile floor is level and strong enough so it will not crack.

Remember that plaster lath walls generate a LOT of debris waste when removed. Figure one cubic foot for each square foot of wall. Its heavy, may contain asbestos, and will create a dust bowl like you never saw before. So be prepared for that. Get a small section of plaster wall tested before you cut it all out and find your home is contaminated with asbestos. Check with local yellow pages for testing labs in your area.

Before you start anything however, Be sure to know what your getting yourself into. Know what type of floor your going to need to place down how thick it is going to be once done. What kind of tub or shower your going to use, Toilet and sink. They all have rough in measurements that are critical in order to properly install them. Having this info and knowing how thick the walls and floors are going to end up being before you start will determine where the pipes go and where the electrial is going to be.

Even more Are you going to use a surface mounted medicine cabinet, or flush mount? Flush will require cutting more holes in the wall and framing. Then If you decide to remove the walls, dont forget the repairs to the other side of the wall. As you will be beating the walls with hammers and saws to remove the plaster. You will most likely crack the plaster on the other side which will need repairs.
What will be disturbed below the new bath. Will you need to cut into the ceiling below or walls below for drains, supply pipes, electrical? Are you running the drains down the bearing wall? How will you get the pipe past the support beam without cutting it in the basement.
Be sure to read and understand proper plumbing drain installations so you will know just where to cut and run the drain pipes.

Do not forget your building permits! As a home inspector, we often find homes that are being sold with baths installed without permits. That can create a lot of headaches for both the owner and the buyer. Any new renovation raises flags for inspectors and the first thing we write in our reports is to obtain permit documents from the seller. Also no permits no insurance. Insurance companies will look into this if a claim is made because of a trip injury in a bath. If they find that the bathroom is done without permits, good luck collecting.

Once you researched this you will then get an understanding why contractors charge what they do. Cheep prices do not take into fact what needs to be done properly. This is why shows like Mike Holmes is in existance. Some contractors quote a job only to find out they did not charge enough to do it. Then cut corners to get it done. Its not an easy undertaking and quite often no understood, but if you do your homework, take your time. Its very rewarding to do it yourself!