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 $12.57   12.387%   $14.127   $14.13   $14.15   $14.10  
  Purchase #2 $36.98   6.771%   $39.4839   $39.48   $39.50   $39.50  
  Purchase #3 $106.41   8.162%   $115.0952   $115.10   $115.10   $115.10  
  Purchase #4 $9.78   7.173%   $10.4815   $10.48   $10.50   $10.50  
  Purchase #5 $124.77   13.126%   $141.1473   $141.15   $141.15   $141.10  
  Purchase #6 $81.44   8.306%   $88.2044   $88.20   $88.20   $88.20  
  Purchase #7 $236.92   4.856%   $248.4248   $248.42   $248.40   $248.40  
  7 purchase totals = $608.87       $656.9641   $656.96   $657.00   $656.90