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 $73.88   8.781%   $80.3674   $80.37   $80.35   $80.40  
  Purchase #2 $7.59   6.180%   $8.0591   $8.06   $8.05   $8.10  
  Purchase #3 $85.41   8.790%   $92.9175   $92.92   $92.90   $92.90  
  Purchase #4 $36.59   4.366%   $38.1875   $38.19   $38.20   $38.20  
  Purchase #5 $194.87   11.313%   $216.9156   $216.92   $216.90   $216.90  
  Purchase #6 $176.37   4.131%   $183.6558   $183.66   $183.65   $183.70  
  Purchase #7 $21.87   11.715%   $24.4321   $24.43   $24.45   $24.40  
  7 purchase totals = $596.58       $644.5350   $644.55   $644.50   $644.60