I'm a beginnger triathlete...The tubes on my bike have been on for a
while. Should I change them before a race? And if water temp is in the
low 70's (air temp will be mid to high 60's), is a wetsuit an absolute?
What's "awhile"? On tubes I pretty much don't change them until they go flat or when I change a tire and pinch the tube and then it goes flat. But I'm no expert on tires so don't blame me if they go flat in the middle of your race. If they are a couple of years old then it might be a good idea to replace them.
Wetsuits are never an absolute. They help you float and keep you warm. Personally I don't think low 70's is that cold. How good of a swimmer are you? Wetsuits helps everyone but the lousier the swimmer the more the wetsuit helps. Lately I haven't been wearing a wetsuit for shorter swims because it takes longer to get it off than the speed benifit. That's my $0.02.