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 $115.70   12.693%   $130.3858   $130.39   $130.40   $130.40  
  Purchase #2 $96.59   6.500%   $102.8684   $102.87   $102.85   $102.90  
  Purchase #3 $94.25   10.200%   $103.8635   $103.86   $103.85   $103.90  
  Purchase #4 $195.57   3.180%   $201.7891   $201.79   $201.80   $201.80  
  Purchase #5 $201.98   10.266%   $222.7153   $222.72   $222.70   $222.70  
  Purchase #6 $75.38   4.379%   $78.6809   $78.68   $78.70   $78.70  
  Purchase #7 $139.41   8.155%   $150.7789   $150.78   $150.80   $150.80  
  7 purchase totals = $918.88       $991.0819   $991.09   $991.10   $991.20