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 $230.28   4.589%   $240.8475   $240.85   $240.85   $240.80  
  Purchase #2 $123.87   4.123%   $128.9772   $128.98   $129.00   $129.00  
  Purchase #3 $33.92   6.632%   $36.1696   $36.17   $36.15   $36.20  
  Purchase #4 $12.37   7.787%   $13.3333   $13.33   $13.35   $13.30  
  Purchase #5 $22.51   4.245%   $23.4655   $23.47   $23.45   $23.50  
  Purchase #6 $171.41   7.315%   $183.9486   $183.95   $183.95   $183.90  
  Purchase #7 $40.40   13.432%   $45.8265   $45.83   $45.85   $45.80  
  7 purchase totals = $634.76       $672.5682   $672.58   $672.60   $672.50