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 $128.97   12.428%   $144.9984   $145.00   $145.00   $145.00  
  Purchase #2 $209.43   3.270%   $216.2784   $216.28   $216.30   $216.30  
  Purchase #3 $230.79   6.576%   $245.9668   $245.97   $245.95   $246.00  
  Purchase #4 $8.81   10.597%   $9.7436   $9.74   $9.75   $9.70  
  Purchase #5 $42.52   12.802%   $47.9634   $47.96   $47.95   $48.00  
  Purchase #6 $217.74   13.576%   $247.3004   $247.30   $247.30   $247.30  
  Purchase #7 $53.37   7.856%   $57.5627   $57.56   $57.55   $57.60  
  7 purchase totals = $891.63       $969.8137   $969.81   $969.80   $969.90