No, you shouldn't. Actually, you mustn't.
EDIT/EXPLANATION: In formal writing, it is not permitted as scopes of theoretically synonimical words may not actually overlap in various fields of science. That is why it is not uncommon to begin a paper with a chapter on terms used and their definitions.
In real life situations, whether you say (of top of my head) contract or agreement is irrelevant. In writing, on which readers will rely for precision, and which may be the basis for their further writing, legal actions, scientific research, etc. it's your duty to be univocal.
Plus, if you use "interchangable" terms in your writing, and it ever gets published, and if the publication ever gets translated, you'll give the translator a real hard time. Not to mention readers abroad.
If you fear boring, repetitive sentences, don't. If what you're writing is coherent, relevant for the reader, and to the point, you'll be fine. Or try recasting your sentences, which usually means they become shorter=clearer=better.