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 $149.79   6.786%   $159.9547   $159.95   $159.95   $160.00  
  Purchase #2 $103.36   11.461%   $115.2061   $115.21   $115.20   $115.20  
  Purchase #3 $186.44   12.449%   $209.6499   $209.65   $209.65   $209.60  
  Purchase #4 $224.95   10.290%   $248.0974   $248.10   $248.10   $248.10  
  Purchase #5 $175.43   10.820%   $194.4115   $194.41   $194.40   $194.40  
  Purchase #6 $25.86   11.801%   $28.9117   $28.91   $28.90   $28.90  
  Purchase #7 $133.34   6.651%   $142.2084   $142.21   $142.20   $142.20  
  7 purchase totals = $999.17       $1,098.4397   $1,098.44   $1,098.40   $1,098.40