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 $44.92   10.825%   $49.7826   $49.78   $49.80   $49.80  
  Purchase #2 $21.61   13.440%   $24.5144   $24.51   $24.50   $24.50  
  Purchase #3 $69.31   8.468%   $75.1792   $75.18   $75.20   $75.20  
  Purchase #4 $49.50   8.331%   $53.6238   $53.62   $53.60   $53.60  
  Purchase #5 $153.72   9.503%   $168.328   $168.33   $168.35   $168.30  
  Purchase #6 $125.40   7.829%   $135.2176   $135.22   $135.20   $135.20  
  Purchase #7 $153.84   5.315%   $162.0166   $162.02   $162.00   $162.00  
  7 purchase totals = $618.30       $668.6622   $668.66   $668.65   $668.60