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 $12.53   10.866%   $13.8915   $13.89   $13.90   $13.90  
  Purchase #2 $87.65   3.585%   $90.7923   $90.79   $90.80   $90.80  
  Purchase #3 $9.54   11.824%   $10.668   $10.67   $10.65   $10.70  
  Purchase #4 $61.64   10.475%   $68.0968   $68.10   $68.10   $68.10  
  Purchase #5 $81.56   10.408%   $90.0488   $90.05   $90.05   $90.00  
  Purchase #6 $149.55   9.577%   $163.8724   $163.87   $163.85   $163.90  
  Purchase #7 $243.32   11.746%   $271.9004   $271.90   $271.90   $271.90  
  7 purchase totals = $645.79       $709.2702   $709.27   $709.25   $709.30