I have a Garmin and they use a similar system, the drain on my Garmin mount was so small most of my equipment could not measure it.
I have hooked my system directly to the battery and also use it to charge my phone and Bluetooth head set while parked.
How often do you ride your bike ? if it is more than once a month you should never have a problem but if you park it up over winter you may want to pull the fuse to stop what could be very small drain.
The best thing to do is measure the drain from your Tomtom and then make a decision.
Some of the 19 rides in my shed
2014 V2, Snoopy
2009 Wee, Pumbaa the pig
WR450F, The Blue Postie Bike
YZ250N smoker with rego, Stinky.
Yamaha MT09 (FZ09), The Scud missile.
Club Lead not Club Med.
He with the most toys wins.
Out of my depth in a puddle.
Live life on the edge you will see more that way.
Ridding a motorcycle keeps things in balance.
At the end of each trail and at the end of each day history is made.