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 $56.79   7.837%   $61.2406   $61.24   $61.25   $61.20  
  Purchase #2 $93.84   11.469%   $104.6025   $104.60   $104.60   $104.60  
  Purchase #3 $197.65   13.428%   $224.1904   $224.19   $224.20   $224.20  
  Purchase #4 $75.84   5.837%   $80.2668   $80.27   $80.25   $80.30  
  Purchase #5 $67.93   8.423%   $73.6517   $73.65   $73.65   $73.70  
  Purchase #6 $143.62   6.427%   $152.8505   $152.85   $152.85   $152.90  
  Purchase #7 $118.78   4.683%   $124.3425   $124.34   $124.35   $124.30  
  7 purchase totals = $754.45       $821.1450   $821.14   $821.15   $821.20