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 $18.56   9.733%   $20.3664   $20.37   $20.35   $20.40  
  Purchase #2 $36.35   8.435%   $39.4161   $39.42   $39.40   $39.40  
  Purchase #3 $103.42   13.558%   $117.4417   $117.44   $117.45   $117.40  
  Purchase #4 $128.58   12.588%   $144.7657   $144.77   $144.75   $144.80  
  Purchase #5 $137.76   12.563%   $155.0668   $155.07   $155.05   $155.10  
  Purchase #6 $63.97   12.709%   $72.0999   $72.10   $72.10   $72.10  
  Purchase #7 $141.34   8.230%   $152.9723   $152.97   $152.95   $153.00  
  7 purchase totals = $629.98       $702.1289   $702.14   $702.05   $702.20