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 $24.52   8.118%   $26.5105   $26.51   $26.50   $26.50  
  Purchase #2 $5.80   7.723%   $6.2479   $6.25   $6.25   $6.20  
  Purchase #3 $19.44   6.722%   $20.7468   $20.75   $20.75   $20.70  
  Purchase #4 $45.98   3.823%   $47.7378   $47.74   $47.75   $47.70  
  Purchase #5 $189.74   10.464%   $209.5944   $209.59   $209.60   $209.60  
  Purchase #6 $84.99   7.735%   $91.564   $91.56   $91.55   $91.60  
  Purchase #7 $225.47   9.485%   $246.8558   $246.86   $246.85   $246.90  
  7 purchase totals = $595.94       $649.2572   $649.26   $649.25   $649.20