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 $29.46   5.154%   $30.9784   $30.98   $31.00   $31.00  
  Purchase #2 $79.67   4.465%   $83.2273   $83.23   $83.25   $83.20  
  Purchase #3 $62.97   7.200%   $67.5038   $67.50   $67.50   $67.50  
  Purchase #4 $17.79   10.808%   $19.7127   $19.71   $19.70   $19.70  
  Purchase #5 $31.28   7.582%   $33.6516   $33.65   $33.65   $33.70  
  Purchase #6 $140.72   9.610%   $154.2432   $154.24   $154.25   $154.20  
  Purchase #7 $72.56   7.163%   $77.7575   $77.76   $77.75   $77.80  
  7 purchase totals = $434.45       $467.0745   $467.07   $467.10   $467.10