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 $12.69   5.431%   $13.3792   $13.38   $13.40   $13.40  
  Purchase #2 $109.46   12.678%   $123.3373   $123.34   $123.35   $123.30  
  Purchase #3 $84.29   9.657%   $92.4299   $92.43   $92.45   $92.40  
  Purchase #4 $122.97   4.185%   $128.1163   $128.12   $128.10   $128.10  
  Purchase #5 $164.66   3.691%   $170.7376   $170.74   $170.75   $170.70  
  Purchase #6 $15.59   13.319%   $17.6664   $17.67   $17.65   $17.70  
  Purchase #7 $117.45   13.296%   $133.0662   $133.07   $133.05   $133.10  
  7 purchase totals = $627.11       $678.7329   $678.75   $678.75   $678.70