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 $110.78   9.341%   $121.128   $121.13   $121.15   $121.10  
  Purchase #2 $121.57   12.748%   $137.0677   $137.07   $137.05   $137.10  
  Purchase #3 $196.65   7.795%   $211.9789   $211.98   $212.00   $212.00  
  Purchase #4 $9.62   5.202%   $10.1204   $10.12   $10.10   $10.10  
  Purchase #5 $16.98   10.660%   $18.7901   $18.79   $18.80   $18.80  
  Purchase #6 $128.89   12.435%   $144.9175   $144.92   $144.90   $144.90  
  Purchase #7 $33.97   7.171%   $36.406   $36.41   $36.40   $36.40  
  7 purchase totals = $618.46       $680.4086   $680.42   $680.40   $680.40