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 $248.99   6.549%   $265.2964   $265.30   $265.30   $265.30  
  Purchase #2 $221.84   11.254%   $246.8059   $246.81   $246.80   $246.80  
  Purchase #3 $171.41   4.137%   $178.5012   $178.50   $178.50   $178.50  
  Purchase #4 $65.32   9.120%   $71.2772   $71.28   $71.30   $71.30  
  Purchase #5 $19.36   4.237%   $20.1803   $20.18   $20.20   $20.20  
  Purchase #6 $79.71   7.424%   $85.6277   $85.63   $85.65   $85.60  
  Purchase #7 $144.94   11.623%   $161.7864   $161.79   $161.80   $161.80  
  7 purchase totals = $951.57       $1,029.4751   $1,029.49   $1,029.55   $1,029.50