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 $44.45   13.161%   $50.3001   $50.30   $50.30   $50.30  
  Purchase #2 $21.67   9.355%   $23.6972   $23.70   $23.70   $23.70  
  Purchase #3 $110.79   8.344%   $120.0343   $120.03   $120.05   $120.00  
  Purchase #4 $202.75   12.280%   $227.6477   $227.65   $227.65   $227.60  
  Purchase #5 $80.74   12.565%   $90.885   $90.89   $90.90   $90.90  
  Purchase #6 $220.75   13.102%   $249.6727   $249.67   $249.65   $249.70  
  Purchase #7 $207.87   7.482%   $223.4228   $223.42   $223.40   $223.40  
  7 purchase totals = $889.02       $985.6598   $985.66   $985.65   $985.60