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 $8.72   12.718%   $9.829   $9.83   $9.85   $9.80  
  Purchase #2 $39.28   4.709%   $41.1297   $41.13   $41.15   $41.10  
  Purchase #3 $85.54   9.110%   $93.3327   $93.33   $93.35   $93.30  
  Purchase #4 $192.98   6.380%   $205.2921   $205.29   $205.30   $205.30  
  Purchase #5 $81.27   9.309%   $88.8354   $88.84   $88.85   $88.80  
  Purchase #6 $213.26   12.832%   $240.6255   $240.63   $240.65   $240.60  
  Purchase #7 $122.82   11.856%   $137.3815   $137.38   $137.40   $137.40  
  7 purchase totals = $743.87       $816.4259   $816.43   $816.55   $816.30