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 $75.65   5.136%   $79.5354   $79.54   $79.55   $79.50  
  Purchase #2 $72.62   10.413%   $80.1819   $80.18   $80.20   $80.20  
  Purchase #3 $85.99   3.625%   $89.1071   $89.11   $89.10   $89.10  
  Purchase #4 $138.55   9.415%   $151.5945   $151.59   $151.60   $151.60  
  Purchase #5 $9.87   7.577%   $10.6178   $10.62   $10.60   $10.60  
  Purchase #6 $232.83   7.401%   $250.0617   $250.06   $250.05   $250.10  
  Purchase #7 $17.61   12.687%   $19.8442   $19.84   $19.85   $19.80  
  7 purchase totals = $633.12       $680.9426   $680.94   $680.95   $680.90