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 $51.82   12.725%   $58.4141   $58.41   $58.40   $58.40  
  Purchase #2 $18.51   8.736%   $20.127   $20.13   $20.15   $20.10  
  Purchase #3 $6.58   5.135%   $6.9179   $6.92   $6.90   $6.90  
  Purchase #4 $212.64   7.590%   $228.7794   $228.78   $228.80   $228.80  
  Purchase #5 $45.72   10.530%   $50.5343   $50.53   $50.55   $50.50  
  Purchase #6 $243.77   5.738%   $257.7575   $257.76   $257.75   $257.80  
  Purchase #7 $15.32   9.526%   $16.7794   $16.78   $16.80   $16.80  
  7 purchase totals = $594.36       $639.3096   $639.31   $639.35   $639.30