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 $172.71   8.130%   $186.7513   $186.75   $186.75   $186.80  
  Purchase #2 $139.31   7.501%   $149.7596   $149.76   $149.75   $149.80  
  Purchase #3 $149.27   10.782%   $165.3643   $165.36   $165.35   $165.40  
  Purchase #4 $13.91   6.684%   $14.8397   $14.84   $14.85   $14.80  
  Purchase #5 $46.88   7.729%   $50.5034   $50.50   $50.50   $50.50  
  Purchase #6 $48.25   5.104%   $50.7127   $50.71   $50.70   $50.70  
  Purchase #7 $20.26   7.224%   $21.7236   $21.72   $21.70   $21.70  
  7 purchase totals = $590.59       $639.6546   $639.64   $639.60   $639.70