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 $9.77   3.864%   $10.1475   $10.15   $10.15   $10.10  
  Purchase #2 $18.74   6.694%   $19.9945   $19.99   $20.00   $20.00  
  Purchase #3 $187.38   9.843%   $205.8238   $205.82   $205.80   $205.80  
  Purchase #4 $130.51   5.516%   $137.7089   $137.71   $137.70   $137.70  
  Purchase #5 $120.71   5.156%   $126.9338   $126.93   $126.95   $126.90  
  Purchase #6 $140.93   7.167%   $151.0305   $151.03   $151.05   $151.00  
  Purchase #7 $108.62   12.490%   $122.1866   $122.19   $122.20   $122.20  
  7 purchase totals = $716.66       $773.8256   $773.82   $773.85   $773.70