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 $28.47   7.698%   $30.6616   $30.66   $30.65   $30.70  
  Purchase #2 $90.54   10.434%   $99.9869   $99.99   $100.00   $100.00  
  Purchase #3 $231.31   12.208%   $259.5483   $259.55   $259.55   $259.50  
  Purchase #4 $114.58   4.417%   $119.641   $119.64   $119.65   $119.60  
  Purchase #5 $85.84   9.731%   $94.1931   $94.19   $94.20   $94.20  
  Purchase #6 $144.59   8.381%   $156.7081   $156.71   $156.70   $156.70  
  Purchase #7 $54.54   7.563%   $58.6649   $58.66   $58.65   $58.70  
  7 purchase totals = $749.87       $819.4039   $819.40   $819.40   $819.40