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 $250.75   10.571%   $277.2568   $277.26   $277.25   $277.30  
  Purchase #2 $86.37   10.226%   $95.2022   $95.20   $95.20   $95.20  
  Purchase #3 $5.60   6.813%   $5.9815   $5.98   $6.00   $6.00  
  Purchase #4 $144.96   13.851%   $165.0384   $165.04   $165.05   $165.00  
  Purchase #5 $17.72   4.442%   $18.5071   $18.51   $18.50   $18.50  
  Purchase #6 $200.74   3.307%   $207.3785   $207.38   $207.40   $207.40  
  Purchase #7 $32.34   11.340%   $36.0074   $36.01   $36.00   $36.00  
  7 purchase totals = $738.48       $805.3719   $805.38   $805.40   $805.40