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 $36.75   9.789%   $40.3475   $40.35   $40.35   $40.30  
  Purchase #2 $16.67   13.784%   $18.9678   $18.97   $18.95   $19.00  
  Purchase #3 $185.56   7.675%   $199.8017   $199.80   $199.80   $199.80  
  Purchase #4 $110.64   13.263%   $125.3142   $125.31   $125.30   $125.30  
  Purchase #5 $87.79   12.284%   $98.5741   $98.57   $98.55   $98.60  
  Purchase #6 $38.62   11.186%   $42.94   $42.94   $42.95   $42.90  
  Purchase #7 $106.60   10.350%   $117.6331   $117.63   $117.65   $117.60  
  7 purchase totals = $582.63       $643.5784   $643.57   $643.55   $643.50