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 $11.47   12.400%   $12.8923   $12.89   $12.90   $12.90  
  Purchase #2 $87.31   13.521%   $99.1152   $99.12   $99.10   $99.10  
  Purchase #3 $129.79   12.430%   $145.9229   $145.92   $145.90   $145.90  
  Purchase #4 $37.62   8.443%   $40.7963   $40.80   $40.80   $40.80  
  Purchase #5 $77.98   10.297%   $86.0096   $86.01   $86.00   $86.00  
  Purchase #6 $150.71   10.675%   $166.7983   $166.80   $166.80   $166.80  
  Purchase #7 $126.74   10.674%   $140.2682   $140.27   $140.25   $140.30  
  7 purchase totals = $621.62       $691.8028   $691.81   $691.75   $691.80