Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $248.75   10.363%   $274.528   $274.53   $274.55   $274.50  
  Purchase #2 $190.72   12.186%   $213.9611   $213.96   $213.95   $214.00  
  Purchase #3 $104.39   8.695%   $113.4667   $113.47   $113.45   $113.50  
  Purchase #4 $17.52   11.580%   $19.5488   $19.55   $19.55   $19.50  
  Purchase #5 $62.48   6.307%   $66.4206   $66.42   $66.40   $66.40  
  Purchase #6 $43.79   5.125%   $46.0342   $46.03   $46.05   $46.00  
  Purchase #7 $231.78   7.545%   $249.2678   $249.27   $249.25   $249.30  
  7 purchase totals = $899.43       $983.2272   $983.23   $983.20   $983.20