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 $6.47   13.129%   $7.3194   $7.32   $7.30   $7.30  
  Purchase #2 $163.72   12.799%   $184.6745   $184.67   $184.65   $184.70  
  Purchase #3 $183.90   7.331%   $197.3817   $197.38   $197.40   $197.40  
  Purchase #4 $200.62   10.317%   $221.318   $221.32   $221.30   $221.30  
  Purchase #5 $52.90   4.483%   $55.2715   $55.27   $55.25   $55.30  
  Purchase #6 $91.78   12.226%   $103.001   $103.00   $103.00   $103.00  
  Purchase #7 $20.80   9.347%   $22.7442   $22.74   $22.75   $22.70  
  7 purchase totals = $720.19       $791.7103   $791.70   $791.65   $791.70