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 $185.64   13.276%   $210.2856   $210.29   $210.30   $210.30  
  Purchase #2 $53.70   6.817%   $57.3607   $57.36   $57.35   $57.40  
  Purchase #3 $81.84   7.597%   $88.0574   $88.06   $88.05   $88.10  
  Purchase #4 $194.91   4.617%   $203.909   $203.91   $203.90   $203.90  
  Purchase #5 $200.47   10.621%   $221.7619   $221.76   $221.75   $221.80  
  Purchase #6 $123.41   3.685%   $127.9577   $127.96   $127.95   $128.00  
  Purchase #7 $14.98   13.102%   $16.9427   $16.94   $16.95   $16.90  
  7 purchase totals = $854.95       $926.2750   $926.28   $926.25   $926.40