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 $147.32   12.233%   $165.3417   $165.34   $165.35   $165.30  
  Purchase #2 $67.57   5.183%   $71.0722   $71.07   $71.05   $71.10  
  Purchase #3 $181.79   13.307%   $205.9808   $205.98   $206.00   $206.00  
  Purchase #4 $175.39   12.443%   $197.2138   $197.21   $197.20   $197.20  
  Purchase #5 $59.33   8.522%   $64.3861   $64.39   $64.40   $64.40  
  Purchase #6 $76.25   12.138%   $85.5052   $85.51   $85.50   $85.50  
  Purchase #7 $116.95   13.484%   $132.7195   $132.72   $132.70   $132.70  
  7 purchase totals = $824.60       $922.2193   $922.22   $922.20   $922.20