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 $249.34   9.827%   $273.8426   $273.84   $273.85   $273.80  
  Purchase #2 $31.74   4.189%   $33.0696   $33.07   $33.05   $33.10  
  Purchase #3 $190.34   5.811%   $201.4007   $201.40   $201.40   $201.40  
  Purchase #4 $232.33   9.540%   $254.4943   $254.49   $254.50   $254.50  
  Purchase #5 $155.84   5.102%   $163.791   $163.79   $163.80   $163.80  
  Purchase #6 $147.42   3.442%   $152.4942   $152.49   $152.50   $152.50  
  Purchase #7 $161.45   12.555%   $181.72   $181.72   $181.70   $181.70  
  7 purchase totals = $1,168.46       $1,260.8124   $1,260.80   $1,260.80   $1,260.80