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 $249.42   7.286%   $267.5927   $267.59   $267.60   $267.60  
  Purchase #2 $179.88   6.772%   $192.0615   $192.06   $192.05   $192.10  
  Purchase #3 $171.51   8.522%   $186.1261   $186.13   $186.15   $186.10  
  Purchase #4 $25.56   9.362%   $27.9529   $27.95   $27.95   $28.00  
  Purchase #5 $55.64   13.344%   $63.0646   $63.06   $63.05   $63.10  
  Purchase #6 $213.57   11.252%   $237.6009   $237.60   $237.60   $237.60  
  Purchase #7 $21.55   13.735%   $24.5099   $24.51   $24.50   $24.50  
  7 purchase totals = $917.13       $998.9086   $998.90   $998.90   $999.00