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 $87.77   3.470%   $90.8156   $90.82   $90.80   $90.80  
  Purchase #2 $52.43   9.720%   $57.5262   $57.53   $57.55   $57.50  
  Purchase #3 $214.48   13.669%   $243.7973   $243.80   $243.80   $243.80  
  Purchase #4 $7.65   12.263%   $8.5881   $8.59   $8.60   $8.60  
  Purchase #5 $136.86   13.344%   $155.1226   $155.12   $155.10   $155.10  
  Purchase #6 $35.80   4.278%   $37.3315   $37.33   $37.35   $37.30  
  Purchase #7 $193.82   12.344%   $217.7451   $217.75   $217.75   $217.70  
  7 purchase totals = $728.81       $810.9264   $810.94   $810.95   $810.80