Oncor, the state’s largest distribution utility which covers Dallas, Fort Worth and much of North Texas, has already agreed to pass all of the millions of dollars of expected tax savings along to consumers.  Oncor agreed to pass the savings along to customers as part of a rate review which is a formal process in which the PUC reviews the appropriateness of rates being charged by the utility.  No exact details have been determined with respect to how the savings will be passed along. The rate review was actually completed before the tax reform bill was passed but there was a commitment in principle to passing along the savings.  It’s not yet know exactly how much Oncor will save from the lower corporate tax rates but with a $245 million tax bill in 2017 future saving are likely to be in the tens of millions of dollars.
The cheapest rates are going to have shorter contract terms. For instance, a 3 month term is cheaper than a 6 month term and a 6 month term is cheaper than a 12-14 month term. So if you are able to remember to call before your contract expires (they will notify via mail you that your contract is about to expire), then the 3 or 6-month term is your best bet. If you don't call to renew/cancel, you will be charged a much higher variable rate on a month-to-month basis. If this is you, sign for a 12-month contract term with the cheapest rate.
On the other hand, month-to-month variable rate (no-contract) plans don’t have cancellation fees. You won’t be penalized if you find a better deal elsewhere and want to make another switch.  And, you won’t be stuck paying more than you should be if the market rate for electricity trends down.  But, if it goes up, you’ll be paying more than your in-contract neighbors, and you’ll likely want to shop around again for a better deal.