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 $243.84   13.805%   $277.5021   $277.50   $277.50   $277.50  
  Purchase #2 $20.49   4.312%   $21.3735   $21.37   $21.35   $21.40  
  Purchase #3 $195.91   11.144%   $217.7422   $217.74   $217.75   $217.70  
  Purchase #4 $106.87   12.190%   $119.8975   $119.90   $119.90   $119.90  
  Purchase #5 $209.63   12.229%   $235.2657   $235.27   $235.25   $235.30  
  Purchase #6 $58.39   3.257%   $60.2918   $60.29   $60.30   $60.30  
  Purchase #7 $33.89   3.578%   $35.1026   $35.10   $35.10   $35.10  
  7 purchase totals = $869.02       $967.1754   $967.17   $967.15   $967.20