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 $124.99   11.298%   $139.1114   $139.11   $139.10   $139.10  
  Purchase #2 $196.26   13.815%   $223.3733   $223.37   $223.35   $223.40  
  Purchase #3 $243.79   11.637%   $272.1598   $272.16   $272.15   $272.20  
  Purchase #4 $249.90   11.839%   $279.4857   $279.49   $279.50   $279.50  
  Purchase #5 $151.78   10.534%   $167.7685   $167.77   $167.75   $167.80  
  Purchase #6 $105.74   11.157%   $117.5374   $117.54   $117.55   $117.50  
  Purchase #7 $123.46   5.270%   $129.9663   $129.97   $129.95   $130.00  
  7 purchase totals = $1,195.92       $1,329.4024   $1,329.41   $1,329.35   $1,329.50