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 $159.87   12.144%   $179.2846   $179.28   $179.30   $179.30  
  Purchase #2 $210.65   4.671%   $220.4895   $220.49   $220.50   $220.50  
  Purchase #3 $55.84   13.381%   $63.312   $63.31   $63.30   $63.30  
  Purchase #4 $216.60   8.181%   $234.32   $234.32   $234.30   $234.30  
  Purchase #5 $109.71   13.198%   $124.1895   $124.19   $124.20   $124.20  
  Purchase #6 $132.72   13.813%   $151.0526   $151.05   $151.05   $151.10  
  Purchase #7 $227.47   13.868%   $259.0155   $259.02   $259.00   $259.00  
  7 purchase totals = $1,112.86       $1,231.6637   $1,231.66   $1,231.65   $1,231.70