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 $125.70   8.181%   $135.9835   $135.98   $136.00   $136.00  
  Purchase #2 $106.67   12.436%   $119.9355   $119.94   $119.95   $119.90  
  Purchase #3 $204.59   10.792%   $226.6694   $226.67   $226.65   $226.70  
  Purchase #4 $236.49   4.748%   $247.7185   $247.72   $247.70   $247.70  
  Purchase #5 $152.60   10.200%   $168.1652   $168.17   $168.15   $168.20  
  Purchase #6 $111.67   8.483%   $121.143   $121.14   $121.15   $121.10  
  Purchase #7 $168.75   8.435%   $182.9841   $182.98   $183.00   $183.00  
  7 purchase totals = $1,106.47       $1,202.5992   $1,202.60   $1,202.60   $1,202.60