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 $241.32   13.621%   $274.1902   $274.19   $274.20   $274.20  
  Purchase #2 $133.93   11.231%   $148.9717   $148.97   $148.95   $149.00  
  Purchase #3 $161.43   7.765%   $173.965   $173.97   $173.95   $174.00  
  Purchase #4 $231.49   13.513%   $262.7712   $262.77   $262.75   $262.80  
  Purchase #5 $23.70   8.752%   $25.7742   $25.77   $25.75   $25.80  
  Purchase #6 $200.37   6.177%   $212.7469   $212.75   $212.75   $212.70  
  Purchase #7 $39.67   11.835%   $44.3649   $44.36   $44.35   $44.40  
  7 purchase totals = $1,031.91       $1,142.7841   $1,142.78   $1,142.70   $1,142.90