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 $20.43   3.233%   $21.0905   $21.09   $21.10   $21.10  
  Purchase #2 $180.66   4.872%   $189.4618   $189.46   $189.45   $189.50  
  Purchase #3 $93.79   10.237%   $103.3913   $103.39   $103.40   $103.40  
  Purchase #4 $166.77   13.311%   $188.9688   $188.97   $188.95   $189.00  
  Purchase #5 $27.49   5.174%   $28.9123   $28.91   $28.90   $28.90  
  Purchase #6 $71.70   3.322%   $74.0819   $74.08   $74.10   $74.10  
  Purchase #7 $30.32   5.270%   $31.9179   $31.92   $31.90   $31.90  
  7 purchase totals = $591.16       $637.8245   $637.82   $637.80   $637.90