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 $30.40   4.777%   $31.8522   $31.85   $31.85   $31.90  
  Purchase #2 $140.99   6.338%   $149.9259   $149.93   $149.95   $149.90  
  Purchase #3 $225.51   11.811%   $252.145   $252.15   $252.15   $252.10  
  Purchase #4 $214.41   13.357%   $243.0487   $243.05   $243.05   $243.00  
  Purchase #5 $198.27   10.861%   $219.8041   $219.80   $219.80   $219.80  
  Purchase #6 $39.56   8.150%   $42.7841   $42.78   $42.80   $42.80  
  Purchase #7 $155.88   8.218%   $168.6902   $168.69   $168.70   $168.70  
  7 purchase totals = $1,005.02       $1,108.2502   $1,108.25   $1,108.30   $1,108.20