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 $25.32   9.744%   $27.7872   $27.79   $27.80   $27.80  
  Purchase #2 $93.96   10.109%   $103.4584   $103.46   $103.45   $103.50  
  Purchase #3 $16.59   11.707%   $18.5322   $18.53   $18.55   $18.50  
  Purchase #4 $133.36   3.270%   $137.7209   $137.72   $137.70   $137.70  
  Purchase #5 $115.46   6.118%   $122.5238   $122.52   $122.50   $122.50  
  Purchase #6 $97.63   11.466%   $108.8243   $108.82   $108.80   $108.80  
  Purchase #7 $113.31   6.138%   $120.265   $120.27   $120.25   $120.30  
  7 purchase totals = $595.63       $639.1118   $639.11   $639.05   $639.10