replication question - connections on slave

Hi

I have a replicated slave copy of my database. Our 6 year old - non modular
web app does not have the connection selector for our PHP scripts, so PHP
opens SQL queries to the default (last connection) made by the scripts. So
if i open a connection to the slave somewhere, it would become the latest
connection and would break write-queries that defaults to it.

I was wondering if there is a downfall to opening the slave connection
first then master, so master would be default. But if there is no read
queries on that page i specially direct to slave, the slave connection
would be opened for nothing and simply close after script executes. Is this
a horrible practice in terms of performance?

thanks