You worry too much. Either size fuse will blow quickly if you accidentally create a short circuit. You wouldn't believe how many 25s and 15's I've blown in my wifes car due to a loose dome light... none came anywhere near melting or burning anything but the fuse.
The only hazard you have comes from UNFUSED circuits straight from the battery. That guy with the 240 radio surely had something like that... and it's stupid on his part. A friend did have a 740 sedan burn to the ground due to bad battery cables shorting together- again, unfused power.
There's no need for the tach to be on a circuit that is on all the time either; there are spare circuits in most 240s with proper capacity that turn on with the key. You really should spend $30 and get a good voltmeter- it will pay dividends on this and many other jobs.
--
Rob Bareiss, New London CT ::: '87 244DL/M47- 229K, 88 744GLE- 218K, 82 245T-181K Also responsible for the care and feeding of: 88 745GLE, 231K, 87 244DL, 239K, 88 245DL, 246K