Preface: I apologize if you already know this info.
Simple answer: The general rule of thumb is "optimal" tube performance (longevity vs tone) is when tubes idle at ~ 70% of maximum plate dissipation.
The ratings of the tubes you mention should be safe, ASSUMING these numbers are derived when the tubes are tested at the same plate voltages as YOUR amp. Power tube pairings should be matched to within 5% of each other with respect to idle current draw. I've seen two problems when buying tubes. First, many on-line sellers (eBay, etc) only "match" the tubes with respect to mutual transconductance, which tells nothing about how much current the tube draws. Transconductance is a measure of how much life the tube has remaining. Second, when tubes are matched with current draw, the plate voltages of the testing unit don't match my amp. But when you buy from a reputable, knowledgeable dealer, they know your amp and can supply tubes that are matched TO YOUR SPECIFIC AMP.
The general rule of thumb is "optimal" tube performance (longevity vs tone) is when tubes idle at ~ 70% of maximum plate dissipation. For instance, EL34s are rated at 25 watts. If your amp draws 475 plate volts at idle, the max dissipation is 53 ma, and 70% would be 37 ma, so that is the max you'd want the tubes to draw when idling at 475 plate volts. In an adjustable bias amp, one would simply adjust the bias until the tubes are drawing the 70% number, like this:
http://www.eurotubes.com/eurotubes-Generic-How-To-Bias-Video-Marshall.htm
Using the above example, in a Mesa FIXED BIAS, the tubes become the variable because there is no bias adjustment. You'll want tubes that idle =< 37ma. Anything below that is cold; above that is hot. Above 53 (100%) can be dangerous or will burn through tubes quickly. For instance, DC-3s have EL84s, which are rated at 12 watts. The DC-3 runs 400 plate volts, so the dissipation is calculated as 12 / 400 = 30 ma, at 70% = 21. So ideally I'd look for tubes that draw 21 ma at idle because I cant adjust the bias unless I have an adjustable bias installed. But EL84s don't come that "cold", and 25 is about the coldest I can get, so the DC-3 runs at ~ 83% dissipation (25/30 = 83%), meaning it is hard on tubes, but sounds very good!
I hope this helps.
