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 $22.97   12.489%   $25.8387   $25.84   $25.85   $25.80  
  Purchase #2 $222.76   11.315%   $247.9653   $247.97   $247.95   $248.00  
  Purchase #3 $249.46   10.613%   $275.9352   $275.94   $275.95   $275.90  
  Purchase #4 $80.92   12.596%   $91.1127   $91.11   $91.10   $91.10  
  Purchase #5 $22.65   8.360%   $24.5435   $24.54   $24.55   $24.50  
  Purchase #6 $18.86   4.787%   $19.7628   $19.76   $19.75   $19.80  
  Purchase #7 $26.53   7.119%   $28.4187   $28.42   $28.40   $28.40  
  7 purchase totals = $644.15       $713.5769   $713.58   $713.55   $713.50