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 $31.31   9.151%   $34.1752   $34.18   $34.20   $34.20  
  Purchase #2 $38.61   3.758%   $40.061   $40.06   $40.05   $40.10  
  Purchase #3 $76.28   9.748%   $83.7158   $83.72   $83.70   $83.70  
  Purchase #4 $231.87   10.192%   $255.5022   $255.50   $255.50   $255.50  
  Purchase #5 $79.84   6.453%   $84.9921   $84.99   $85.00   $85.00  
  Purchase #6 $40.84   3.511%   $42.2739   $42.27   $42.25   $42.30  
  Purchase #7 $113.43   6.579%   $120.8926   $120.89   $120.90   $120.90  
  7 purchase totals = $612.18       $661.6128   $661.61   $661.60   $661.70