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 $20.50   4.570%   $21.4369   $21.44   $21.45   $21.40  
  Purchase #2 $14.26   6.270%   $15.1541   $15.15   $15.15   $15.20  
  Purchase #3 $66.48   7.848%   $71.6974   $71.70   $71.70   $71.70  
  Purchase #4 $70.57   12.360%   $79.2925   $79.29   $79.30   $79.30  
  Purchase #5 $248.54   12.416%   $279.3987   $279.40   $279.40   $279.40  
  Purchase #6 $223.97   3.111%   $230.9377   $230.94   $230.95   $230.90  
  Purchase #7 $15.31   12.437%   $17.2141   $17.21   $17.20   $17.20  
  7 purchase totals = $659.63       $715.1314   $715.13   $715.15   $715.10