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 $204.95   8.238%   $221.8338   $221.83   $221.85   $221.80  
  Purchase #2 $203.56   8.763%   $221.398   $221.40   $221.40   $221.40  
  Purchase #3 $131.55   6.290%   $139.8245   $139.82   $139.80   $139.80  
  Purchase #4 $21.28   11.115%   $23.6453   $23.65   $23.65   $23.60  
  Purchase #5 $7.28   9.511%   $7.9724   $7.97   $7.95   $8.00  
  Purchase #6 $162.92   8.320%   $176.4749   $176.47   $176.45   $176.50  
  Purchase #7 $107.99   9.276%   $118.0072   $118.01   $118.00   $118.00  
  7 purchase totals = $839.53       $909.1561   $909.15   $909.10   $909.10