Sichuanese: When Is First Tone 55 Vs. 45?
When you are talking about 成都话, the first tone (阴平) follows the following rules:

1) The regular case for the first tone is 45. When you are reading a single character, you should use 45.

2) When the character is part of a phrase or a sentence, it may change. Specifically, when a first tone character A is preceded by another first tone character B, A is changed to 55. For example, 区分 is pronounced qu(45) fen(55).

In addition, the default tone values for the second tone (阳平), the third tone (上声) and the fourth tone (去声) are 21, 42 and 13, respectively. These are used when you read a single character. When they are combined with other characters, the following rules are followed:

3) If a second-stone character is followed by any other characters, its tone value is changed to 22 (from 21). For example, 可以 should be pronounced ke(22)yi(42). Still, when you hear recordings, you may not hear the flattening. So I feel this rule is not strict. 

4) If a third-tone character is followed by any other characters, its tone value is changed to 44 (from 42). For example, 口音 should be read as kou(44)yin(55) by this rule. However, when listening to recordings, again I hear something like kou(43)yin(55). The falling tone is reduced but is still discernible. Further, the first tone of 音 is also read as 55 here.

You can find the above pronunciation yourself in the video: https://www.youtube.com/watch?v=FtauJBczNwA

In summary, I think there are two forms of tone values. One is the standard form, and another is a reduced form used in compound words. It should just come naturally.

The origin Chinese reference for this is:

成都话中，阴平、阳平、上声、去声的调值分别为45, 21, 42, 13。在单读一个字时，音调遵循以上所列。但在词汇与语句中连读时，会有一定规律的变调。

一个阴平字之前一个字读阴平时，本字变调为55。

一个阳平字之后有任何字时，本字变调为22。

一个上声字之后有任何字时，本字变调为44。

Reference: http://zh.wikipedia.org/wiki/%E6%88%90%E9%83%BD%E8%AF%9D