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 $45.39   6.777%   $48.4661   $48.47   $48.45   $48.50  
  Purchase #2 $184.73   7.133%   $197.9068   $197.91   $197.90   $197.90  
  Purchase #3 $156.52   6.581%   $166.8206   $166.82   $166.80   $166.80  
  Purchase #4 $170.75   5.274%   $179.7554   $179.76   $179.75   $179.80  
  Purchase #5 $74.39   6.489%   $79.2172   $79.22   $79.20   $79.20  
  Purchase #6 $250.82   4.332%   $261.6855   $261.69   $261.70   $261.70  
  Purchase #7 $58.42   11.380%   $65.0682   $65.07   $65.05   $65.10  
  7 purchase totals = $941.02       $998.9198   $998.94   $998.85   $999.00