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 $31.95   8.775%   $34.7536   $34.75   $34.75   $34.80  
  Purchase #2 $16.28   9.659%   $17.8525   $17.85   $17.85   $17.90  
  Purchase #3 $187.90   12.272%   $210.9591   $210.96   $210.95   $211.00  
  Purchase #4 $57.80   9.275%   $63.161   $63.16   $63.15   $63.20  
  Purchase #5 $168.83   6.808%   $180.3239   $180.32   $180.30   $180.30  
  Purchase #6 $125.32   6.485%   $133.447   $133.45   $133.45   $133.40  
  Purchase #7 $23.93   7.146%   $25.64   $25.64   $25.65   $25.60  
  7 purchase totals = $612.01       $666.1371   $666.13   $666.10   $666.20