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 $78.65   12.288%   $88.3145   $88.31   $88.30   $88.30  
  Purchase #2 $219.67   12.234%   $246.5444   $246.54   $246.55   $246.50  
  Purchase #3 $22.80   8.407%   $24.7168   $24.72   $24.70   $24.70  
  Purchase #4 $111.40   7.343%   $119.5801   $119.58   $119.60   $119.60  
  Purchase #5 $247.86   6.691%   $264.4443   $264.44   $264.45   $264.40  
  Purchase #6 $62.28   4.358%   $64.9942   $64.99   $65.00   $65.00  
  Purchase #7 $30.64   4.225%   $31.9345   $31.93   $31.95   $31.90  
  7 purchase totals = $773.30       $840.5288   $840.51   $840.55   $840.40