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 $123.80   9.712%   $135.8235   $135.82   $135.80   $135.80  
  Purchase #2 $155.40   12.440%   $174.7318   $174.73   $174.75   $174.70  
  Purchase #3 $77.41   9.211%   $84.5402   $84.54   $84.55   $84.50  
  Purchase #4 $84.69   6.607%   $90.2855   $90.29   $90.30   $90.30  
  Purchase #5 $16.47   9.867%   $18.0951   $18.10   $18.10   $18.10  
  Purchase #6 $40.88   11.128%   $45.4291   $45.43   $45.45   $45.40  
  Purchase #7 $229.63   12.779%   $258.9744   $258.97   $258.95   $259.00  
  7 purchase totals = $728.28       $807.8796   $807.88   $807.90   $807.80