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 $246.99   6.595%   $263.279   $263.28   $263.30   $263.30  
  Purchase #2 $18.39   11.787%   $20.5576   $20.56   $20.55   $20.60  
  Purchase #3 $176.96   4.182%   $184.3605   $184.36   $184.35   $184.40  
  Purchase #4 $249.93   3.695%   $259.1649   $259.16   $259.15   $259.20  
  Purchase #5 $98.44   12.495%   $110.7401   $110.74   $110.75   $110.70  
  Purchase #6 $33.48   11.423%   $37.3044   $37.30   $37.30   $37.30  
  Purchase #7 $112.67   4.707%   $117.9734   $117.97   $117.95   $118.00  
  7 purchase totals = $936.86       $993.3799   $993.37   $993.35   $993.50