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 $246.67   5.262%   $259.6498   $259.65   $259.65   $259.60  
  Purchase #2 $43.53   12.409%   $48.9316   $48.93   $48.95   $48.90  
  Purchase #3 $177.50   4.616%   $185.6934   $185.69   $185.70   $185.70  
  Purchase #4 $31.79   13.380%   $36.0435   $36.04   $36.05   $36.00  
  Purchase #5 $189.58   9.324%   $207.2564   $207.26   $207.25   $207.30  
  Purchase #6 $217.52   11.196%   $241.8735   $241.87   $241.85   $241.90  
  Purchase #7 $30.46   13.324%   $34.5185   $34.52   $34.50   $34.50  
  7 purchase totals = $937.05       $1,013.9667   $1,013.96   $1,013.95   $1,013.90