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 $171.87   9.757%   $188.6394   $188.64   $188.65   $188.60  
  Purchase #2 $43.34   7.210%   $46.4648   $46.46   $46.45   $46.50  
  Purchase #3 $237.99   4.453%   $248.5877   $248.59   $248.60   $248.60  
  Purchase #4 $25.97   13.713%   $29.5313   $29.53   $29.55   $29.50  
  Purchase #5 $58.53   3.397%   $60.5183   $60.52   $60.50   $60.50  
  Purchase #6 $203.73   12.439%   $229.072   $229.07   $229.05   $229.10  
  Purchase #7 $184.53   8.488%   $200.1929   $200.19   $200.20   $200.20  
  7 purchase totals = $925.96       $1,003.0064   $1,003.00   $1,003.00   $1,003.00