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 $100.43   9.446%   $109.9166   $109.92   $109.90   $109.90  
  Purchase #2 $227.41   12.501%   $255.8385   $255.84   $255.85   $255.80  
  Purchase #3 $130.47   8.870%   $142.0427   $142.04   $142.05   $142.00  
  Purchase #4 $110.75   9.536%   $121.3111   $121.31   $121.30   $121.30  
  Purchase #5 $198.98   11.102%   $221.0708   $221.07   $221.05   $221.10  
  Purchase #6 $15.87   3.273%   $16.3894   $16.39   $16.40   $16.40  
  Purchase #7 $54.39   6.494%   $57.9221   $57.92   $57.90   $57.90  
  7 purchase totals = $838.30       $924.4912   $924.49   $924.45   $924.40