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 $110.27   9.277%   $120.4997   $120.50   $120.50   $120.50  
  Purchase #2 $221.47   10.843%   $245.484   $245.48   $245.50   $245.50  
  Purchase #3 $225.84   7.629%   $243.0693   $243.07   $243.05   $243.10  
  Purchase #4 $29.42   13.778%   $33.4735   $33.47   $33.45   $33.50  
  Purchase #5 $164.77   3.376%   $170.3326   $170.33   $170.35   $170.30  
  Purchase #6 $31.96   11.231%   $35.5494   $35.55   $35.55   $35.50  
  Purchase #7 $8.70   3.276%   $8.985   $8.99   $9.00   $9.00  
  7 purchase totals = $792.43       $857.3935   $857.39   $857.40   $857.40