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 $57.55   10.569%   $63.6325   $63.63   $63.65   $63.60  
  Purchase #2 $79.47   4.828%   $83.3068   $83.31   $83.30   $83.30  
  Purchase #3 $203.39   4.251%   $212.0361   $212.04   $212.05   $212.00  
  Purchase #4 $75.34   6.381%   $80.1474   $80.15   $80.15   $80.10  
  Purchase #5 $245.72   5.450%   $259.1117   $259.11   $259.10   $259.10  
  Purchase #6 $160.49   4.762%   $168.1325   $168.13   $168.15   $168.10  
  Purchase #7 $100.30   10.216%   $110.5466   $110.55   $110.55   $110.50  
  7 purchase totals = $922.26       $976.9136   $976.92   $976.95   $976.70