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 $28.62   10.313%   $31.5716   $31.57   $31.55   $31.60  
  Purchase #2 $228.54   3.872%   $237.3891   $237.39   $237.40   $237.40  
  Purchase #3 $37.31   3.349%   $38.5595   $38.56   $38.55   $38.60  
  Purchase #4 $5.63   7.435%   $6.0486   $6.05   $6.05   $6.00  
  Purchase #5 $63.84   12.186%   $71.6195   $71.62   $71.60   $71.60  
  Purchase #6 $75.86   5.350%   $79.9185   $79.92   $79.90   $79.90  
  Purchase #7 $83.38   13.686%   $94.7914   $94.79   $94.80   $94.80  
  7 purchase totals = $523.18       $559.8982   $559.90   $559.85   $559.90