maybe this might work

client.c

@@ -243,6 +243,8 @@ manage(Window w, XWindowAttributes *wa)
 		c->isfloat = trans
 			|| (c->maxw && c->minw &&
 				c->maxw == c->minw && c->maxh == c->minh);
+	if(c->isfloat)
+		c->weight = ntags;
 
 	if(clients)
 		clients->prev = c;

config.mk

@@ -1,5 +1,5 @@
 # dwm version
-VERSION = 1.4
+VERSION = 1.5
 
 # Customize below to fit your system
 

tag.c

@@ -110,7 +110,8 @@ settags(Client *c, Client *trans)
 	if(!matched)
 		for(i = 0; i < ntags; i++)
 			c->tags[i] = seltag[i];
-	for(c->weight = 0; c->weight < ntags && !c->tags[c->weight]; c->weight++);
+	if(!c->isfloat)
+		for(c->weight = 0; c->weight < ntags && !c->tags[c->weight]; c->weight++);
 }
 
 void
@@ -124,7 +125,8 @@ tag(Arg *arg)
 	for(i = 0; i < ntags; i++)
 		sel->tags[i] = False;
 	sel->tags[arg->i] = True;
-	sel->weight = arg->i;
+	if(!sel->isfloat)
+		sel->weight = arg->i;
 	arrange(NULL);
 }
 
@@ -140,6 +142,7 @@ toggletag(Arg *arg)
 	for(i = 0; i < ntags && !sel->tags[i]; i++);
 	if(i == ntags)
 		sel->tags[arg->i] = True;
-	sel->weight = (i == ntags) ? arg->i : i;
+	if(!sel->isfloat)
+		sel->weight = (i == ntags) ? arg->i : i;
 	arrange(NULL);
 }