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 $7.87   7.749%   $8.4798   $8.48   $8.50   $8.50  
  Purchase #2 $13.84   10.426%   $15.283   $15.28   $15.30   $15.30  
  Purchase #3 $165.41   8.501%   $179.4715   $179.47   $179.45   $179.50  
  Purchase #4 $28.30   9.621%   $31.0227   $31.02   $31.00   $31.00  
  Purchase #5 $232.86   5.147%   $244.8453   $244.85   $244.85   $244.80  
  Purchase #6 $174.51   11.131%   $193.9347   $193.93   $193.95   $193.90  
  Purchase #7 $129.60   6.236%   $137.6819   $137.68   $137.70   $137.70  
  7 purchase totals = $752.39       $810.7189   $810.71   $810.75   $810.70