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 $249.80   8.682%   $271.4876   $271.49   $271.50   $271.50  
  Purchase #2 $237.51   6.172%   $252.1691   $252.17   $252.15   $252.20  
  Purchase #3 $213.43   5.249%   $224.6329   $224.63   $224.65   $224.60  
  Purchase #4 $147.33   12.446%   $165.6667   $165.67   $165.65   $165.70  
  Purchase #5 $76.46   4.699%   $80.0529   $80.05   $80.05   $80.10  
  Purchase #6 $167.96   8.639%   $182.4701   $182.47   $182.45   $182.50  
  Purchase #7 $183.50   5.759%   $194.0678   $194.07   $194.05   $194.10  
  7 purchase totals = $1,275.99       $1,370.5471   $1,370.55   $1,370.50   $1,370.70