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 $123.28   10.734%   $136.5129   $136.51   $136.50   $136.50  
  Purchase #2 $211.93   11.656%   $236.6326   $236.63   $236.65   $236.60  
  Purchase #3 $224.50   4.702%   $235.056   $235.06   $235.05   $235.10  
  Purchase #4 $188.97   4.842%   $198.1199   $198.12   $198.10   $198.10  
  Purchase #5 $165.98   7.394%   $178.2526   $178.25   $178.25   $178.30  
  Purchase #6 $116.66   3.208%   $120.4025   $120.40   $120.40   $120.40  
  Purchase #7 $238.79   9.253%   $260.8852   $260.89   $260.90   $260.90  
  7 purchase totals = $1,270.11       $1,365.8617   $1,365.86   $1,365.85   $1,365.90