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 $240.39   7.571%   $258.5899   $258.59   $258.60   $258.60  
  Purchase #2 $126.88   13.283%   $143.7335   $143.73   $143.75   $143.70  
  Purchase #3 $170.74   8.424%   $185.1231   $185.12   $185.10   $185.10  
  Purchase #4 $191.33   11.759%   $213.8285   $213.83   $213.85   $213.80  
  Purchase #5 $244.30   10.544%   $270.059   $270.06   $270.05   $270.10  
  Purchase #6 $236.54   4.265%   $246.6284   $246.63   $246.65   $246.60  
  Purchase #7 $80.65   5.815%   $85.3398   $85.34   $85.35   $85.30  
  7 purchase totals = $1,290.83       $1,403.3022   $1,403.30   $1,403.35   $1,403.20