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 $250.35   13.433%   $283.9795   $283.98   $284.00   $284.00  
  Purchase #2 $67.81   12.649%   $76.3873   $76.39   $76.40   $76.40  
  Purchase #3 $115.85   10.430%   $127.9332   $127.93   $127.95   $127.90  
  Purchase #4 $25.26   4.646%   $26.4336   $26.43   $26.45   $26.40  
  Purchase #5 $243.95   9.287%   $266.6056   $266.61   $266.60   $266.60  
  Purchase #6 $54.80   11.407%   $61.051   $61.05   $61.05   $61.10  
  Purchase #7 $226.36   4.562%   $236.6865   $236.69   $236.70   $236.70  
  7 purchase totals = $984.38       $1,079.0767   $1,079.08   $1,079.15   $1,079.10