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 $177.53   6.163%   $188.4712   $188.47   $188.45   $188.50  
  Purchase #2 $24.47   12.189%   $27.4526   $27.45   $27.45   $27.50  
  Purchase #3 $189.51   3.124%   $195.4303   $195.43   $195.45   $195.40  
  Purchase #4 $81.60   12.475%   $91.7796   $91.78   $91.80   $91.80  
  Purchase #5 $66.26   3.513%   $68.5877   $68.59   $68.60   $68.60  
  Purchase #6 $44.87   8.484%   $48.6768   $48.68   $48.70   $48.70  
  Purchase #7 $63.33   8.397%   $68.6478   $68.65   $68.65   $68.60  
  7 purchase totals = $647.57       $689.0460   $689.05   $689.10   $689.10