There are no "best practices". Only things that make sense and make things easier.

Extracting the common parts and parameterizing the rest is one such thing:

lines="`gunzip -c /var/log/cisco/cisco.log-$(date +%Y%m%d).gz`"
#gunzip would always output the same thing on the same day, so 
#just run it once and store the results in a variable
grepAndLog(){
  local regex="$1" file="$2" msg="$3" filter="${4:-cat}"
  #^names for positional parameters

  printf "%s\n" "$lines" | grep "$regex" | cut -d' ' -f4 | sort | uniq -c | eval "$filter"  > "/tmp/$file"
  local count=`wc -l < "/tmp/$file"`   
  echo "There are $count "" $msg" >> "/tmp/$file"
}
grepAndLog ath_bstuck_tasklet netgear_bacon.txt \
 'Stuck beacon; resetting'
grepAndLog netgear_buffer_queue netgear_buffer_queue.txt \
 "routers with 'Virtual device ath0 asks to queue packet' errors"
grepAndLog CMS_MSG_DNSPROXY_RELOAD netgear_dns.txt \
 " routers with 'DNS Proxy Issue' errors"
grepAndLog ath_bstuck_tasklet netgear_bacon.txt \
 " routers with beacon frame errors"
grepAndLog ACK netgear_ACK.txt \
 " routers with more than 50 ACK" 'awk -v x=50 "\$1 >= x"'

It's still a mainly-shell solution. But IMO more readable and over 40% shorter.

About the code:

I'm using grep "$regex" | cut -d' ' -f4 instead of the awk expression. Other than that the grepAndLog function is a generalization of what you do in each line of your script: You have some input (the output of gunzip), you grep that for an expression (the $regex parameter), and output the resulting lines, sorted and prefixed with count into a $file. Then you append the line count (I do wc -l < "$file" instead of wc -l "$file" | awk ...) wrapped in a message whose beginning is constant and whose end varies ($msg).

In your last line you don't simply grep, but you use another filter on top of that. Instead of creating an if branch for that in the function, I simply use cat as an implicit default additional filter in the normal cases where no fourth parameter exists (local filter="${4:-cat}" means create a function-local variable filter whose contents is the fourth parameter given to the function, or cat if no fourth parameter is provided). cat gets overriden if a fourth parameter is given to grepAndLog.