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 $238.30   11.173%   $264.9253   $264.93   $264.95   $264.90  
  Purchase #2 $108.84   8.300%   $117.8737   $117.87   $117.85   $117.90  
  Purchase #3 $170.51   5.122%   $179.2435   $179.24   $179.25   $179.20  
  Purchase #4 $133.95   3.119%   $138.1279   $138.13   $138.15   $138.10  
  Purchase #5 $49.39   9.342%   $54.004   $54.00   $54.00   $54.00  
  Purchase #6 $26.82   11.376%   $29.871   $29.87   $29.85   $29.90  
  Purchase #7 $22.92   7.439%   $24.625   $24.63   $24.65   $24.60  
  7 purchase totals = $750.73       $808.6704   $808.67   $808.70   $808.60