For a day game with the roof open, the best seats out of the sun are in Rows 35 and higher in the lower level infield bowl - especially sections 116-144. These higher rows are fully covered and do not rely on shadows to keep you cool. Additionally, the number of steps from the concourse to these high rows is minimal.
If stairs are not a burden, Rows 15-25 in Sections 334-344 are a cheaper alternative. These seats are covered by the roof at all times and will have the sun behind them during the late afternoon and evening.
Finally, Rows 10-12 in the 200 level seats are covered from above and offer relief from the sun.