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 $103.57   12.156%   $116.16   $116.16   $116.15   $116.20  
  Purchase #2 $66.27   12.853%   $74.7877   $74.79   $74.80   $74.80  
  Purchase #3 $70.45   12.814%   $79.4775   $79.48   $79.50   $79.50  
  Purchase #4 $31.80   9.162%   $34.7135   $34.71   $34.70   $34.70  
  Purchase #5 $247.87   6.710%   $264.5021   $264.50   $264.50   $264.50  
  Purchase #6 $41.97   5.607%   $44.3233   $44.32   $44.30   $44.30  
  Purchase #7 $92.44   3.346%   $95.533   $95.53   $95.55   $95.50  
  7 purchase totals = $654.37       $709.4971   $709.49   $709.50   $709.50