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 $83.64   12.156%   $93.8073   $93.81   $93.80   $93.80  
  Purchase #2 $35.33   9.323%   $38.6238   $38.62   $38.60   $38.60  
  Purchase #3 $54.26   13.365%   $61.5118   $61.51   $61.50   $61.50  
  Purchase #4 $237.30   6.412%   $252.5157   $252.52   $252.50   $252.50  
  Purchase #5 $120.56   10.328%   $133.0114   $133.01   $133.00   $133.00  
  Purchase #6 $110.41   12.533%   $124.2477   $124.25   $124.25   $124.20  
  Purchase #7 $182.90   10.212%   $201.5777   $201.58   $201.60   $201.60  
  7 purchase totals = $824.40       $905.2954   $905.30   $905.25   $905.20