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 $226.80   9.793%   $249.0105   $249.01   $249.00   $249.00  
  Purchase #2 $35.42   6.859%   $37.8495   $37.85   $37.85   $37.80  
  Purchase #3 $177.32   12.862%   $200.1269   $200.13   $200.15   $200.10  
  Purchase #4 $142.45   7.868%   $153.658   $153.66   $153.65   $153.70  
  Purchase #5 $16.63   7.654%   $17.9029   $17.90   $17.90   $17.90  
  Purchase #6 $60.80   5.745%   $64.293   $64.29   $64.30   $64.30  
  Purchase #7 $46.83   7.848%   $50.5052   $50.51   $50.50   $50.50  
  7 purchase totals = $706.25       $773.3460   $773.35   $773.35   $773.30