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 $229.77   10.764%   $254.5024   $254.50   $254.50   $254.50  
  Purchase #2 $106.56   3.838%   $110.6498   $110.65   $110.65   $110.60  
  Purchase #3 $237.90   5.782%   $251.6554   $251.66   $251.65   $251.70  
  Purchase #4 $12.48   7.440%   $13.4085   $13.41   $13.40   $13.40  
  Purchase #5 $154.81   7.412%   $166.2845   $166.28   $166.30   $166.30  
  Purchase #6 $165.26   5.237%   $173.9147   $173.91   $173.90   $173.90  
  Purchase #7 $84.72   13.513%   $96.1682   $96.17   $96.15   $96.20  
  7 purchase totals = $991.50       $1,066.5835   $1,066.58   $1,066.55   $1,066.60