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 $223.41   12.244%   $250.7643   $250.76   $250.75   $250.80  
  Purchase #2 $168.59   7.164%   $180.6678   $180.67   $180.65   $180.70  
  Purchase #3 $69.59   7.663%   $74.9227   $74.92   $74.90   $74.90  
  Purchase #4 $13.43   8.208%   $14.5323   $14.53   $14.55   $14.50  
  Purchase #5 $169.28   12.128%   $189.8103   $189.81   $189.80   $189.80  
  Purchase #6 $164.41   3.369%   $169.949   $169.95   $169.95   $169.90  
  Purchase #7 $135.29   10.310%   $149.2384   $149.24   $149.25   $149.20  
  7 purchase totals = $944.00       $1,029.8848   $1,029.88   $1,029.85   $1,029.80