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 $183.37   13.396%   $207.9342   $207.93   $207.95   $207.90  
  Purchase #2 $21.95   12.869%   $24.7747   $24.77   $24.75   $24.80  
  Purchase #3 $160.74   3.129%   $165.7696   $165.77   $165.75   $165.80  
  Purchase #4 $33.33   12.208%   $37.3989   $37.40   $37.40   $37.40  
  Purchase #5 $85.97   11.679%   $96.0104   $96.01   $96.00   $96.00  
  Purchase #6 $126.98   8.517%   $137.7949   $137.79   $137.80   $137.80  
  Purchase #7 $33.67   4.327%   $35.1269   $35.13   $35.15   $35.10  
  7 purchase totals = $646.01       $704.8096   $704.80   $704.80   $704.80