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 $108.92   11.415%   $121.3532   $121.35   $121.35   $121.40  
  Purchase #2 $157.78   12.293%   $177.1759   $177.18   $177.20   $177.20  
  Purchase #3 $70.98   11.770%   $79.3343   $79.33   $79.35   $79.30  
  Purchase #4 $93.36   11.142%   $103.7622   $103.76   $103.75   $103.80  
  Purchase #5 $230.79   6.437%   $245.646   $245.65   $245.65   $245.60  
  Purchase #6 $155.28   6.733%   $165.735   $165.74   $165.75   $165.70  
  Purchase #7 $24.80   6.842%   $26.4968   $26.50   $26.50   $26.50  
  7 purchase totals = $841.91       $919.5034   $919.51   $919.55   $919.50