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 $101.31   7.586%   $108.9954   $109.00   $109.00   $109.00  
  Purchase #2 $80.25   5.516%   $84.6766   $84.68   $84.70   $84.70  
  Purchase #3 $125.37   10.660%   $138.7344   $138.73   $138.75   $138.70  
  Purchase #4 $198.54   13.681%   $225.7023   $225.70   $225.70   $225.70  
  Purchase #5 $27.56   4.467%   $28.7911   $28.79   $28.80   $28.80  
  Purchase #6 $8.74   4.786%   $9.1583   $9.16   $9.15   $9.20  
  Purchase #7 $103.81   12.476%   $116.7613   $116.76   $116.75   $116.80  
  7 purchase totals = $645.58       $712.8194   $712.82   $712.85   $712.90