Date: 10/27/2008 3:44 pm EDT
Having a saw and a grinder is the best, but if you wan't to use just a grinder you will be fine.
Rather than spraying water on the grinder blade make sure you have a good quality blade that cuts porcelain tile dry then you should only need one blade to complete your project.
Instead of buying both a grinder and a saw maybe there is a local rental store you could get a saw for a day, or if you have one available you could use a standard tile cutter to make all your straight cuts.
Hope this helps.