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 $29.72   4.333%   $31.0078   $31.01   $31.00   $31.00  
  Purchase #2 $12.56   3.315%   $12.9764   $12.98   $13.00   $13.00  
  Purchase #3 $37.87   4.303%   $39.4995   $39.50   $39.50   $39.50  
  Purchase #4 $10.34   11.717%   $11.5515   $11.55   $11.55   $11.60  
  Purchase #5 $152.39   4.799%   $159.7032   $159.70   $159.70   $159.70  
  Purchase #6 $138.96   7.812%   $149.8156   $149.82   $149.80   $149.80  
  Purchase #7 $91.69   6.603%   $97.7443   $97.74   $97.75   $97.70  
  7 purchase totals = $473.53       $502.2983   $502.30   $502.30   $502.30