When developing Processing sketches, I often use graph paper to develop ideas --- I find that it's also useful to overlay a grid on a on-going processing sketch -- as a feature request, could this be added to the Processing Core? Also, perhaps an enhancement would be for the IDE to toggle the grid on and off.

here is the code I'm currently using:

void grid(int x, int y, int w, int h, int n, color c) {
 stroke(c);
 for (int ix=x; ix <= x+w; ix += n) {
   line(ix, y, ix, y+h);
 }

 for (int iy=y; iy <= y+h; iy += n) {
   line(x, iy, x+w, iy);
 }
}

void grid(int w, int h, int n) {
 grid(0,0,w,h,n,color(126,50));
}

Good Idea!
I expanded your method to have more options:
Code:

void setup() {
  size(400,400);
  background(153);
  noLoop();
}//setup()

void draw() {
  gridWH(100,100,20);
  gridWH(4,4,130,130,20);
  gridMNC(8,8,4,4,20,color(230,60,100,80));
  gridMNC(110,110,5,9,20,color(100,220,70,180));
}//draw()

//========================================================
// grid of given width/height

void gridWHC(int x0, int y0, int w, int h, int cellw, color c) {
  stroke(c); 
  for (int iy=y0; iy<=y0+h; iy+=cellw) line(x0, iy, x0+w, iy); 
  for (int ix=x0; ix<=x0+w; ix+=cellw) line(ix, y0, ix, y0+h); 
}//gridWHC()

void gridWHC(int w, int h, int cellw, color c) { 
  gridWHC(0,0,w,h,cellw,c); 
}//gridWHC()

void gridWHC(int x0, int y0, int w, int h, color c) { 
  gridWHC(x0,y0,w,h,10,c); 
}//gridWHC()

void gridWHC(int w, int h, color c) { 
  gridWHC(0,0,w,h,10,c); 
}//gridWHC()

void gridWH(int x0, int y0, int w, int h, int cellw) { 
  gridWHC(x0,y0,w,h,cellw,color(20,100,100,80)); 
}//gridWH()

void gridWH(int w, int h, int cellw) { 
  gridWHC(0,0,w,h,cellw,color(20,100,100,80)); 
}//gridWH()

void gridWH(int x0, int y0, int w, int h) { 
  gridWHC(x0,y0,w,h,10,color(20,100,100,80)); 
}//gridWH()

void gridWH(int w, int h) { 
  gridWHC(0,0,w,h,10,color(20,100,100,80)); 
}//gridWH()

//========================================================
// grid of given #row/#column

void gridMNC(int x0, int y0, int mrow, int ncol, int cellw, color c) { 
  stroke(c); 
  int x1=x0+ncol*cellw;
  int y1=y0+mrow*cellw;
  for (int i=0, iy=y0; i<=mrow; i++, iy+=cellw) line(x0, iy, x1, iy);
  for (int i=0, ix=x0; i<=ncol; i++, ix+=cellw) line(ix, y0, ix, y1);
}//gridMNC()

void gridMNC(int mrow, int ncol, int cellw, color c) { 
  gridMNC(0,0,mrow,ncol,cellw,c); 
}//gridMNC()

void gridMNC(int x0, int y0, int mrow, int ncol, color c) { 
  gridMNC(x0,y0,mrow,ncol,10,c); 
}//gridMNC()

void gridMNC(int mrow, int ncol, color c) { 
  gridMNC(0,0,mrow,ncol,10,c); 
}//gridMNC()

void gridMN(int x0, int y0, int mrow, int ncol, int cellw) { 
  gridMNC(x0,y0,mrow,ncol,cellw,color(20,100,100,80)); 
}//gridMN()

void gridMN(int mrow, int ncol, int cellw) { 
  gridMNC(0,0,mrow,ncol,cellw,color(20,100,100,80)); 
}//gridMN()

void gridMN(int x0, int y0, int mrow, int ncol) { 
  gridMNC(x0,y0,mrow,ncol,10,color(20,100,100,80)); 
}//gridMN()

void gridMN(int mrow, int ncol) { 
  gridMNC(0,0,mrow,ncol,10,color(20,100,100,80)); 
}//gridMN()