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 $88.94   10.496%   $98.2751   $98.28   $98.30   $98.30  
  Purchase #2 $50.97   6.405%   $54.2346   $54.23   $54.25   $54.20  
  Purchase #3 $58.64   3.279%   $60.5628   $60.56   $60.55   $60.60  
  Purchase #4 $83.50   12.280%   $93.7538   $93.75   $93.75   $93.80  
  Purchase #5 $51.54   10.371%   $56.8852   $56.89   $56.90   $56.90  
  Purchase #6 $70.88   5.417%   $74.7196   $74.72   $74.70   $74.70  
  Purchase #7 $62.26   6.692%   $66.4264   $66.43   $66.45   $66.40  
  7 purchase totals = $466.73       $504.8575   $504.86   $504.90   $504.90