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 $84.72   10.171%   $93.3369   $93.34   $93.35   $93.30  
  Purchase #2 $132.97   9.229%   $145.2418   $145.24   $145.25   $145.20  
  Purchase #3 $26.80   11.538%   $29.8922   $29.89   $29.90   $29.90  
  Purchase #4 $214.58   10.808%   $237.7718   $237.77   $237.75   $237.80  
  Purchase #5 $81.91   12.720%   $92.329   $92.33   $92.35   $92.30  
  Purchase #6 $108.60   4.782%   $113.7933   $113.79   $113.80   $113.80  
  Purchase #7 $111.47   12.785%   $125.7214   $125.72   $125.70   $125.70  
  7 purchase totals = $761.05       $838.0864   $838.08   $838.10   $838.00