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 $34.49   12.568%   $38.8247   $38.82   $38.80   $38.80  
  Purchase #2 $220.98   9.560%   $242.1057   $242.11   $242.10   $242.10  
  Purchase #3 $115.64   3.361%   $119.5267   $119.53   $119.55   $119.50  
  Purchase #4 $22.65   13.477%   $25.7025   $25.70   $25.70   $25.70  
  Purchase #5 $182.33   11.503%   $203.3034   $203.30   $203.30   $203.30  
  Purchase #6 $138.85   10.458%   $153.3709   $153.37   $153.35   $153.40  
  Purchase #7 $204.75   13.556%   $232.5059   $232.51   $232.50   $232.50  
  7 purchase totals = $919.69       $1,015.3398   $1,015.34   $1,015.30   $1,015.30