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 $238.57   12.665%   $268.7849   $268.78   $268.80   $268.80  
  Purchase #2 $89.27   7.828%   $96.2581   $96.26   $96.25   $96.30  
  Purchase #3 $31.28   4.438%   $32.6682   $32.67   $32.65   $32.70  
  Purchase #4 $249.89   9.644%   $273.9894   $273.99   $274.00   $274.00  
  Purchase #5 $60.97   3.318%   $62.993   $62.99   $63.00   $63.00  
  Purchase #6 $176.93   6.112%   $187.744   $187.74   $187.75   $187.70  
  Purchase #7 $94.98   3.707%   $98.5009   $98.50   $98.50   $98.50  
  7 purchase totals = $941.89       $1,020.9385   $1,020.93   $1,020.95   $1,021.00