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 $52.67   12.742%   $59.3812   $59.38   $59.40   $59.40  
  Purchase #2 $246.49   12.391%   $277.0326   $277.03   $277.05   $277.00  
  Purchase #3 $205.38   13.360%   $232.8188   $232.82   $232.80   $232.80  
  Purchase #4 $136.69   9.377%   $149.5074   $149.51   $149.50   $149.50  
  Purchase #5 $51.30   12.415%   $57.6689   $57.67   $57.65   $57.70  
  Purchase #6 $124.30   13.290%   $140.8195   $140.82   $140.80   $140.80  
  Purchase #7 $7.87   9.371%   $8.6075   $8.61   $8.60   $8.60  
  7 purchase totals = $824.70       $925.8359   $925.84   $925.80   $925.80