The Vpp must be above the level (14 v or so) within 
a certain number of clock pulses. 
When your programming circuit has a slow ( = bad) Vpp raise it 
helps to keep the xtal pins open (the worst it can do is pick up 
50/60 Hz) as compare to leaveing the xtal connected.
Wouter. 

----------
> From: Niklas CarlŽn <niklas.krakau@SWIPNET.SE>
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Programming PICs
> Date: Wednesday, September 22, 1999 09:44
> 
> Hi 
> 
> One quick question:
> Is it OK to leave osc pins floating when programming? 
> Someone told me not to but he can«t remember where he got that advice (or
if it really applied to PICs) and I can«t find anything about this in the
datasheets or programming specs.
> 
> /Niklas CarlŽn